Holographic Materials

**3**

**Chapter 1**

**Abstract**

bleaching

**1. Introduction**

*and Klyukin Dmitry*

Features of Volume Holograms

Photothermorefractive Glass

*Nikonorov Nikolay, Ivanov Sergei, Dubrovin Victor* 

Today, silicate photothermorefractive (PTR) glasses are well known as a holographic medium for fabrication of holographic volume diffractive optical elements. The photothermoinduced crystallization process is used for recording high-efficiency phase volume holograms in this material. These holograms are used for developing unique diffractive optical elements that provide new opportunities for the laser technique, for example, narrowband filters for solid-state lasers and laser diodes, beam combiners, holographic collimator sights, chirped gratings for laser pulse compression, etc. By now, the photothermoinduced crystallization and properties of the PTR glass are investigated well enough. However, there are some issues and features still, which are solved in the present work. The mechanism of refractive index change in fluoride photothermorefractive glass during photothermoinduced crystallization and refractive index profile of the volume Bragg gratings were discussed. We studied a fine structure of a core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell. We report on the optical properties of volume Bragg gratings in chloride PTR glass after femtosecond laser bleaching. We demonstrated that the bleaching procedure significantly reduces the

absorption and increases the thermal stability of the Bragg gratings.

**Keywords:** photothermorefractive glass, holography, Bragg grating, crystallization,

Photothermorefractive (PTR) glasses are a new class of photosensitive materials intended for recording three-dimensional phase holograms. This glass originates from photosensitive sodium zinc aluminosilicate glasses that were applied by Stookey and Pierson in Corning, Inc. (USA) in 1977 for the first time and were referred to as polychromatic glass [1–4] worldwide and by Tsekhomsky in Vavilov State Optical Institute (Russia) [5, 6] as multichromatic (MC) glass. In the late 1980s to early 1990s [7–10], Glebov and Nikonorov in Vavilov State Optical Institute (Russia) were first proposed to implement MC glass for volume holography. This chapter is focused on the photothermoinduced crystallization process itself and the enhancement of the refractive index contrast between irradiated and nonirradiated

in Fluoride and Chloride

#### **Chapter 1**

## Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass

*Nikonorov Nikolay, Ivanov Sergei, Dubrovin Victor and Klyukin Dmitry*

#### **Abstract**

Today, silicate photothermorefractive (PTR) glasses are well known as a holographic medium for fabrication of holographic volume diffractive optical elements. The photothermoinduced crystallization process is used for recording high-efficiency phase volume holograms in this material. These holograms are used for developing unique diffractive optical elements that provide new opportunities for the laser technique, for example, narrowband filters for solid-state lasers and laser diodes, beam combiners, holographic collimator sights, chirped gratings for laser pulse compression, etc. By now, the photothermoinduced crystallization and properties of the PTR glass are investigated well enough. However, there are some issues and features still, which are solved in the present work. The mechanism of refractive index change in fluoride photothermorefractive glass during photothermoinduced crystallization and refractive index profile of the volume Bragg gratings were discussed. We studied a fine structure of a core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell. We report on the optical properties of volume Bragg gratings in chloride PTR glass after femtosecond laser bleaching. We demonstrated that the bleaching procedure significantly reduces the absorption and increases the thermal stability of the Bragg gratings.

**Keywords:** photothermorefractive glass, holography, Bragg grating, crystallization, bleaching

#### **1. Introduction**

Photothermorefractive (PTR) glasses are a new class of photosensitive materials intended for recording three-dimensional phase holograms. This glass originates from photosensitive sodium zinc aluminosilicate glasses that were applied by Stookey and Pierson in Corning, Inc. (USA) in 1977 for the first time and were referred to as polychromatic glass [1–4] worldwide and by Tsekhomsky in Vavilov State Optical Institute (Russia) [5, 6] as multichromatic (MC) glass. In the late 1980s to early 1990s [7–10], Glebov and Nikonorov in Vavilov State Optical Institute (Russia) were first proposed to implement MC glass for volume holography. This chapter is focused on the photothermoinduced crystallization process itself and the enhancement of the refractive index contrast between irradiated and nonirradiated

areas. As a result, a new class of materials was developed, which is denoted [10] by a specific term such as "photothermorefractive (PTR) glass" (i.e., glass wherein after UV irradiation and subsequent thermal treatment changes in refractive index occur). Later this term started to be used worldwide [11, 12].

Nowadays, there is an increased interest in the volume Bragg gratings recorded on PTR glass due to their outstanding properties. The main advantage of these glasses is their unique combination of working characteristics such as high spectral selectivity and operating angel, great mechanical and optical strength, and high chemical durability. A great number of optical elements have been developed based on PTR glass, including extra narrow-band filters, combiners of high-intensity light beams, wavelength division multiplexing (WDM) devices, filters for increasing the spectral brightness of laser diodes, chirped gratings for compressing the light impulses, etc.

By now, the photothermoinduced crystallization and properties of the PTR glass are investigated well enough. For example, the fluoride PTR glass was designed and synthesized in ITMO University, Russia [13]. It is a photosensitive multicomponent sodium-zinc-alumina-silicate glass containing fluorine (6 mol.%) and a small amount of bromine (0.5 mol.%). Also, the PTR glass is doped with cerium, antimony, and silver. These additives are responsible for the photothermoinduced crystallization process resulting in precipitation of silver nanoparticles and sodium fluoride crystals in the volume of the glass [2, 14, 15]. Untreated fluoride PTR glass is transparent in a wide spectral range of 250–2500 nm. The photothermoinduced crystallization process in the fluoride PTR glass consists of three stages: (i) a formation of neutral silver molecular clusters after UV irradiation into the Ce3+ absorption band, (ii) a formation of the silver nanoparticles under the subsequent heat treatment of the UV-irradiated PTR glass near the glass transition temperature (Tg) [2], and (iii) a precipitation of silver bromide shells on the silver nanoparticles [16] and then a growth of sodium fluoride nanocrystals on them in shape of cones under the thermal treatment at temperatures above Tg [2, 17]. The basic optical and spectral properties of PTR glass are described in [2, 15, 18, 19].

In comparison with conventional holographic media (photopolymers, silver halides, gelatins, etc.), the fluoride PTR glass and the holographic optical elements (HOE)s recorded on the glass have outstanding operating characteristics. In particular, they demonstrate the high thermal stability of the HOEs and high resistance to the optical and ionizing irradiation. The optical and spectral parameters of the HOEs and gradient index (GRIN) elements do not change after its multiple heating to high enough temperature (500°C). HOEs recorded on the PTR glass show the high chemical stability and mechanical and optical resistance. From this point of view, the fluoride PTR glass reveals practically no difference with the commercial optical glass BK7 (Schott). The HOEs have a very high duration of life (dozens of years). The key advantages of fluoride PTR glass as a holographic medium are the following:


**5**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

tion are commercially available and not too expensive.

iv. a simple synthesis of fluoride PTR glass allows us to fabricate the glass both in the laboratory conditions (hundreds of grams) and industrial ones (hundreds of kilograms). The chemical reagents required for glass fabrica-

It should be noted that some features of fluoride PTR glass are unusual in comparison with other holographic media. The PTR glass contains sodium ions. These ions can be changed on another monovalent ion (silver, potassium, rubidium, and cesium) by a well-known ion-exchange technique. This approach allowed us to fabricate ion-exchanged optical, luminescent, and plasmonic waveguides as well as

to increase the mechanical, thermal, and optical resistance of the glass [20].

sign of the RI increment by changing the type of halide (fluoride to chloride) in the PTR glass composition. The substitution of fluorine by chlorine resulted in the precipitation of nanocrystalline phases of mixed silver and sodium chlorides in glass host after UV irradiation and thermal treatment, which leads to a positive RI

chloride PTR glass provides the opportunity not only for Bragg gratings but also for

Untreated chloride PTR glass has the same transparency as fluoride one. And the photothermoinduced crystallization process is similar to the fluoride glass. The first stage is a generation of molecular silver clusters during UV irradiation, and the second stage is a formation of silver nanoparticles under heat treatment. On the following third stage, the heat treatment of PTR glass at temperatures above Tg leads to the precipitation of silver nanoparticles with a shell consisting of mixed sodium and silver chlorides [19, 21]. The absorption coefficient of silver nanoparticles in chlo-

is a key disadvantage chloride PTR. Despite this, the chloride PTR glass exhibits a rather low level of scattering. This is due to the small sizes of silver nanoparticles (about 3 nm) and small size of silver and sodium chloride nanocrystals (less 27 nm) [19]. This fact allowed us to conclude that the chloride PTR glass is still very attrac-

Despite the photothermoinduced crystallization and properties of the PTR glass are investigated well enough, there are some issues still, which restrict a wide application of the PTR glass. This chapter is a survey of recent achievements of ITMO University (St. Petersburg, Russia) that are focused on the investigation of the following issues: (i) mechanism of refractive index change in fluoride PTR glass during photothermoinduced crystallization, (ii) refractive index profile of the volume Bragg gratings, (iii) fine structure of core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell, (iv) bleach of volume Bragg gratings in chloride PTR glass for decreasing optical losses and improving their thermal stability.

**2. Mechanism of refractive index change in fluoride PTR glass during** 

According to the calculations presented in [22], the residual stresses are the main reason of RIC in fluoride PTR glass. However, according to Nikonorov et al. [14], this effect is assumed to be due to the difference of refractive indexes of sodium fluoride nanocrystals and PTR glass. Recently, we conducted an experiment that proved

with that of the nonirradiated area (Δn = −1.0 × 10<sup>−</sup><sup>3</sup>

ride PTR glass (at 450 nm) is very high and exceeds 100 cm<sup>−</sup><sup>1</sup>

tive recording medium for photonics applications and for HOEs.

**photothermoinduced crystallization**

changes Δn = +1.0 × 10<sup>−</sup><sup>3</sup>

the optical waveguides recording.

In case of fluoride PTR glass, the photothermoinduced crystallization results in a negative change of the refractive index (RI) in the UV-irradiated area in comparison

. It should be noted that positive refractive index change in

). We managed to control the

. This huge absorption

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

iv. a simple synthesis of fluoride PTR glass allows us to fabricate the glass both in the laboratory conditions (hundreds of grams) and industrial ones (hundreds of kilograms). The chemical reagents required for glass fabrication are commercially available and not too expensive.

It should be noted that some features of fluoride PTR glass are unusual in comparison with other holographic media. The PTR glass contains sodium ions. These ions can be changed on another monovalent ion (silver, potassium, rubidium, and cesium) by a well-known ion-exchange technique. This approach allowed us to fabricate ion-exchanged optical, luminescent, and plasmonic waveguides as well as to increase the mechanical, thermal, and optical resistance of the glass [20].

In case of fluoride PTR glass, the photothermoinduced crystallization results in a negative change of the refractive index (RI) in the UV-irradiated area in comparison with that of the nonirradiated area (Δn = −1.0 × 10<sup>−</sup><sup>3</sup> ). We managed to control the sign of the RI increment by changing the type of halide (fluoride to chloride) in the PTR glass composition. The substitution of fluorine by chlorine resulted in the precipitation of nanocrystalline phases of mixed silver and sodium chlorides in glass host after UV irradiation and thermal treatment, which leads to a positive RI changes Δn = +1.0 × 10<sup>−</sup><sup>3</sup> . It should be noted that positive refractive index change in chloride PTR glass provides the opportunity not only for Bragg gratings but also for the optical waveguides recording.

Untreated chloride PTR glass has the same transparency as fluoride one. And the photothermoinduced crystallization process is similar to the fluoride glass. The first stage is a generation of molecular silver clusters during UV irradiation, and the second stage is a formation of silver nanoparticles under heat treatment. On the following third stage, the heat treatment of PTR glass at temperatures above Tg leads to the precipitation of silver nanoparticles with a shell consisting of mixed sodium and silver chlorides [19, 21]. The absorption coefficient of silver nanoparticles in chloride PTR glass (at 450 nm) is very high and exceeds 100 cm<sup>−</sup><sup>1</sup> . This huge absorption is a key disadvantage chloride PTR. Despite this, the chloride PTR glass exhibits a rather low level of scattering. This is due to the small sizes of silver nanoparticles (about 3 nm) and small size of silver and sodium chloride nanocrystals (less 27 nm) [19]. This fact allowed us to conclude that the chloride PTR glass is still very attractive recording medium for photonics applications and for HOEs.

Despite the photothermoinduced crystallization and properties of the PTR glass are investigated well enough, there are some issues still, which restrict a wide application of the PTR glass. This chapter is a survey of recent achievements of ITMO University (St. Petersburg, Russia) that are focused on the investigation of the following issues: (i) mechanism of refractive index change in fluoride PTR glass during photothermoinduced crystallization, (ii) refractive index profile of the volume Bragg gratings, (iii) fine structure of core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell, (iv) bleach of volume Bragg gratings in chloride PTR glass for decreasing optical losses and improving their thermal stability.

#### **2. Mechanism of refractive index change in fluoride PTR glass during photothermoinduced crystallization**

According to the calculations presented in [22], the residual stresses are the main reason of RIC in fluoride PTR glass. However, according to Nikonorov et al. [14], this effect is assumed to be due to the difference of refractive indexes of sodium fluoride nanocrystals and PTR glass. Recently, we conducted an experiment that proved

*Holographic Materials and Applications*

impulses, etc.

areas. As a result, a new class of materials was developed, which is denoted [10] by a specific term such as "photothermorefractive (PTR) glass" (i.e., glass wherein after UV irradiation and subsequent thermal treatment changes in refractive index

Nowadays, there is an increased interest in the volume Bragg gratings recorded on PTR glass due to their outstanding properties. The main advantage of these glasses is their unique combination of working characteristics such as high spectral selectivity and operating angel, great mechanical and optical strength, and high chemical durability. A great number of optical elements have been developed based on PTR glass, including extra narrow-band filters, combiners of high-intensity light beams, wavelength division multiplexing (WDM) devices, filters for increasing the spectral brightness of laser diodes, chirped gratings for compressing the light

By now, the photothermoinduced crystallization and properties of the PTR glass

are investigated well enough. For example, the fluoride PTR glass was designed and synthesized in ITMO University, Russia [13]. It is a photosensitive multicomponent sodium-zinc-alumina-silicate glass containing fluorine (6 mol.%) and a small amount of bromine (0.5 mol.%). Also, the PTR glass is doped with cerium, antimony, and silver. These additives are responsible for the photothermoinduced crystallization process resulting in precipitation of silver nanoparticles and sodium fluoride crystals in the volume of the glass [2, 14, 15]. Untreated fluoride PTR glass is transparent in a wide spectral range of 250–2500 nm. The photothermoinduced crystallization process in the fluoride PTR glass consists of three stages: (i) a formation of neutral silver molecular clusters after UV irradiation into the Ce3+ absorption band, (ii) a formation of the silver nanoparticles under the subsequent heat treatment of the UV-irradiated PTR glass near the glass transition temperature (Tg) [2], and (iii) a precipitation of silver bromide shells on the silver nanoparticles [16] and then a growth of sodium fluoride nanocrystals on them in shape of cones under the thermal treatment at temperatures above Tg [2, 17]. The basic optical and

In comparison with conventional holographic media (photopolymers, silver halides, gelatins, etc.), the fluoride PTR glass and the holographic optical elements (HOE)s recorded on the glass have outstanding operating characteristics. In particular, they demonstrate the high thermal stability of the HOEs and high resistance to the optical and ionizing irradiation. The optical and spectral parameters of the HOEs and gradient index (GRIN) elements do not change after its multiple heating to high enough temperature (500°C). HOEs recorded on the PTR glass show the high chemical stability and mechanical and optical resistance. From this point of view, the fluoride PTR glass reveals practically no difference with the commercial optical glass BK7 (Schott). The HOEs have a very high duration of life (dozens of years). The key advantages of fluoride PTR glass as a holographic medium are the

i.high optical homogeneity (the refraction index fluctuations across the glass bulk

ii.high reproducibility of parameters of HOEs with high diffraction efficiency

iii.variety of technologies of glass processing—the fluoride PTR glass (like commercial optical glass) can be subjected to various kinds of mechanical grinding and polishing and another shaping technology like pressing, molding, drawing

occur). Later this term started to be used worldwide [11, 12].

spectral properties of PTR glass are described in [2, 15, 18, 19].

),

fiber, as well as laser treatment, and ion exchange,

**4**

following:

are of the order of 10<sup>−</sup><sup>5</sup>

(above 99%),

#### **Figure 1.**

*Refractive index modulation amplitude evolution depending on the sample temperature (photos of the sample before and after the experiment on the insert).*

that the origin of refractive index change in the PTR glass arises from the refractive index difference between PTR glass and NaF crystals.

In the experiment, transmission grating was heated in the chamber up to 600°C with 100°C step, while angular selectivity contour and diffraction efficiency (DE) were measured. Analysis of the selectivity contour revealed that heating up to the 500°C had no measurable effect on the grating DE and therefore on refractive index modulation amplitude (RIMA).

Also, it had been shown that DE of the grating starts to oscillate during the heating procedure as soon as the temperature reaches 480°C, which is due to the crystal growth process that begins around 480°C [21] and leads to RIMA value rapid increase with temperature (**Figure 1**).

Based on the results of our experiment, it is reasonable to conclude that the mechanical residual stresses cannot be the main reason of the RIC in PTR glass as even at 550°C, that is, 80°C higher than Tg, recorded Bragg grating is still functioning. Thus, the main reason for the refractive index contrast between exposed and unexposed areas is the refractive index difference between unperturbed glass and composite with NaF inclusions (**Figure 2**).

To estimate the RIC for this case, Maxwell Garnett theory [30] that allows to calculate the refractive index of the composite medium with the low volume fraction of inclusions had been applied.

To calculate the composite parameters, a model of needle NaF inclusions in the glass matrix had been chosen. In this case, the effective dielectric constant of the composite can be estimated according to the following equation [30]:

$$\delta = \left(\frac{\varepsilon - \varepsilon\_h}{\varepsilon\_l - \varepsilon\_h}\right) \left(\frac{\mathcal{G}\,\varepsilon\_l + \varepsilon\_h}{\mathcal{G}\,\varepsilon + \varepsilon\_h}\right)^{2/5} \tag{1}$$

**7**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

NaF volume fraction was calculated from the TEM image of the grating fringe (**Figure 2(right)**). Estimated difference of the refractive indices of such composite

*Schematic representation of the volume Bragg grating (left) and TEM image of the actual grating fringe (right).*

same order as the RIC in fluoride PTR glass after standard development procedure. For experimental determination of RIC in the grating, five diffraction orders have been measured. Sum of all harmonics equals to the refractive index difference

enough to the value obtained by the optical method. The difference between two methods can be attributed to rough approximations of the NaF crystal surrounding matrix refractive index as well as the fact that heat treatment leads to the changes in

Thus, it is shown that refractive index change in the exposed area appears primarily due to the NaF nanocrystal inclusions in the glass matrix, which according to the Maxwell-Garnet theory decreases the effective refractive index of such composite. It is shown experimentally that residual stresses have no measurable

To record volume Bragg grating with holographic technique, one needs to interfere with two coherent beams inside the photosensitive medium. This interference will create a stationary wave with sinusoidal spatial profile of intensity. Next, a periodic variation of refractive index is inflicted by the process specific for each material. In our case, this process is used to be called photothermoinduced crystallization. Spatial distribution of the crystals results in a spatial modulation of the refractive index. In the case of nonsinusoidal gratings, refractive index profile can be expanded into a Fourier series where each harmonic represents the higher orders of diffraction. And of course, the more orders one sees, the less sinusoidal the gratings are. To utilize the whole refractive index change potential of the PTR glass, one needs to record pure sinusoidal gratings. We study the distribution of the RIC

among the harmonics depending on the exposure dosage (**Figure 3(a)**).

profile, 96.6% of RIC is in the first harmonic (**Figure 4**). It is expected that with an increase of exposure, fraction of the RIC in the first harmonic falls. However, it does not drop below 32% even when exposure doses are enormous like 16 J/cm<sup>2</sup>

It is reasonable to propose that the grating profile may be affected by the scattered light; however, in such case, fraction of the RIC at the first harmonic would fall with exposure linearly. However, **Figure 3(a)** represents the saturation effect. Thus, the fall of the first harmonic fraction is not related to the scattered light during the recording process.

As one can see, gratings with exposure of 0.5 J/cm2

between exposed and unexposed areas and corresponds to 27.92 × 10<sup>−</sup><sup>4</sup>

time, estimations based on Maxwell-Garnet theory gives us 27.62 × 10<sup>−</sup><sup>4</sup>

**3. Refractive index profile of the volume Bragg gratings**

. It is clearly seen that this value is of the

. At the same

have a nearly pure sinusoidal

.

that is close

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

and unperturbed glass gave us 27.62 × 10<sup>−</sup><sup>4</sup>

**Figure 2.**

the refractive index of the unexposed glass [15].

effect on the refractive index change.

where δ is the NaF volume fraction, ε is the effective dielectric constant of a composite, εi is the dielectric constant of the NaF inclusions, and εh is the dielectric constant of the glass matrix.

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

**Figure 2.** *Schematic representation of the volume Bragg grating (left) and TEM image of the actual grating fringe (right).*

NaF volume fraction was calculated from the TEM image of the grating fringe (**Figure 2(right)**). Estimated difference of the refractive indices of such composite and unperturbed glass gave us 27.62 × 10<sup>−</sup><sup>4</sup> . It is clearly seen that this value is of the same order as the RIC in fluoride PTR glass after standard development procedure.

For experimental determination of RIC in the grating, five diffraction orders have been measured. Sum of all harmonics equals to the refractive index difference between exposed and unexposed areas and corresponds to 27.92 × 10<sup>−</sup><sup>4</sup> . At the same time, estimations based on Maxwell-Garnet theory gives us 27.62 × 10<sup>−</sup><sup>4</sup> that is close enough to the value obtained by the optical method. The difference between two methods can be attributed to rough approximations of the NaF crystal surrounding matrix refractive index as well as the fact that heat treatment leads to the changes in the refractive index of the unexposed glass [15].

Thus, it is shown that refractive index change in the exposed area appears primarily due to the NaF nanocrystal inclusions in the glass matrix, which according to the Maxwell-Garnet theory decreases the effective refractive index of such composite. It is shown experimentally that residual stresses have no measurable effect on the refractive index change.

#### **3. Refractive index profile of the volume Bragg gratings**

To record volume Bragg grating with holographic technique, one needs to interfere with two coherent beams inside the photosensitive medium. This interference will create a stationary wave with sinusoidal spatial profile of intensity. Next, a periodic variation of refractive index is inflicted by the process specific for each material. In our case, this process is used to be called photothermoinduced crystallization. Spatial distribution of the crystals results in a spatial modulation of the refractive index. In the case of nonsinusoidal gratings, refractive index profile can be expanded into a Fourier series where each harmonic represents the higher orders of diffraction. And of course, the more orders one sees, the less sinusoidal the gratings are. To utilize the whole refractive index change potential of the PTR glass, one needs to record pure sinusoidal gratings. We study the distribution of the RIC among the harmonics depending on the exposure dosage (**Figure 3(a)**).

As one can see, gratings with exposure of 0.5 J/cm2 have a nearly pure sinusoidal profile, 96.6% of RIC is in the first harmonic (**Figure 4**). It is expected that with an increase of exposure, fraction of the RIC in the first harmonic falls. However, it does not drop below 32% even when exposure doses are enormous like 16 J/cm<sup>2</sup> .

It is reasonable to propose that the grating profile may be affected by the scattered light; however, in such case, fraction of the RIC at the first harmonic would fall with exposure linearly. However, **Figure 3(a)** represents the saturation effect. Thus, the fall of the first harmonic fraction is not related to the scattered light during the recording process.

*Holographic Materials and Applications*

that the origin of refractive index change in the PTR glass arises from the refractive

*Refractive index modulation amplitude evolution depending on the sample temperature (photos of the sample* 

In the experiment, transmission grating was heated in the chamber up to 600°C with 100°C step, while angular selectivity contour and diffraction efficiency (DE) were measured. Analysis of the selectivity contour revealed that heating up to the 500°C had no measurable effect on the grating DE and therefore on refractive index

Also, it had been shown that DE of the grating starts to oscillate during the heating procedure as soon as the temperature reaches 480°C, which is due to the crystal growth process that begins around 480°C [21] and leads to RIMA value rapid

Based on the results of our experiment, it is reasonable to conclude that the mechanical residual stresses cannot be the main reason of the RIC in PTR glass as even at 550°C, that is, 80°C higher than Tg, recorded Bragg grating is still functioning. Thus, the main reason for the refractive index contrast between exposed and unexposed areas is the refractive index difference between unperturbed glass and

To estimate the RIC for this case, Maxwell Garnett theory [30] that allows to calculate the refractive index of the composite medium with the low volume frac-

To calculate the composite parameters, a model of needle NaF inclusions in the glass matrix had been chosen. In this case, the effective dielectric constant of the

> *\_\_\_\_\_ 5 ε<sup>i</sup> + ε<sup>h</sup> 5ε + ε<sup>h</sup>* )

*2/5*

(1)

composite can be estimated according to the following equation [30]:

*\_\_\_\_\_ ε − ε<sup>h</sup> ε<sup>i</sup> − εh*)(

where δ is the NaF volume fraction, ε is the effective dielectric constant of a composite, εi is the dielectric constant of the NaF inclusions, and εh is the dielectric

index difference between PTR glass and NaF crystals.

modulation amplitude (RIMA).

*before and after the experiment on the insert).*

**Figure 1.**

increase with temperature (**Figure 1**).

composite with NaF inclusions (**Figure 2**).

tion of inclusions had been applied.

*δ =* (

constant of the glass matrix.

**6**

**Figure 3.**

*(a) Dependence of the RIC distribution between harmonics on dosage; (b) comparison of refractive index profile in the holographic grating with exposure 0.5 and 16 J/cm2 .*

#### **Figure 4.**

*(a) Absorption spectrum dynamic of the UV irradiated and heated up to 485°C for 90 min fluoride PTR glass. (b) Dynamics of SPR peak location depending on the heat treatment duration for the parent PTR glass and glass with the lowest possible concentration of fluorine.*

In [23], such effect was modeled considering the saturation of the RIC on exposure. Such saturation would lead to broadening of the grating fringes and decreasing the grating contrast. This situation can occur than in the interference pattern, maximum exposure already reached its saturation and thus does not contribute to further increase RIC. In this case, further exposure leads to a decrease in the contrast between the dark and the bright regions in terms of refractive index. However, our experiments show that fringes in the gratings with big exposure are narrower than that in gratings with small exposures **Figure 3(b)**. We suppose that the nonsinusoidal profile is a complex problem and cannot be explained with the saturation of exposure.

#### **4. Fine structure of core-shell system inside fluoride PTR glass**

It is clear that (i) during photothermoinduced (PTI) crystallization process, the silver nanoparticles (NP) precipitate first (ii) that initiate sodium fluoride nanocrystals growth that is responsible for the refractive index changes. At the same time, the period between silver nanoparticles precipitation and sodium fluoride crystals growth is still unclear.

In **Figure 4(a)**, the evolution of the UV-irradiated fluoride PTR glass spectrum in the course of the heating process (heating up to 485°C for 90 min) is shown. It is seen clearly that until 300°C, no significant changes appear in the visible absorption

**9**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

spectrum. Some processes occurring in this temperature range manifest themselves mostly in the UV. Such behavior had been discussed a lot in [18, 24] and attributed to the cerium and antimony ions in different valency. Further heating the sample leads to the formation of silver molecular clusters, thus causing an absorption increase in the visible. After it, when a temperature close to Tg is reached, it is seen that the SPR absorption peak corresponded to silver NPs manifesting in around 400 nm is already formed. When starting the procedure of heat treating the sample, one can observe a significant redshift of the SPR peak location (**Figure 6**). Such behavior of the SPR band can be explained by the occurrence of the shell with a high refractive index on

During the photothermoinduced crystallization process of silver nanoparticles, precipitation occurs first, which is followed by formation of highly refractivity phase (silver bromide) when compared with glass matrix (1.498) [21]. It should be noted that after SPR peak location reaches 460 nm, it starts moving toward the shorter wavelengths up to 450 nm, and there, it remains independently from treatment duration time increase. The blueshift indicates the precipitation of NaF crystals that reduce the refractive index of the core-shell system surrounding. Stabilization of the peak location is due to the fact that NaF crystals already act as hosts for the core-shell structures. In the case of PTR glass without fluorine subjected to the same heat treatment procedure, the only redshift of the SPR resonance peak is observed (fluorinefree glass, **Figure 6(b)**). Hence, it can be concluded that the blue shift is inflicted by

To estimate the size of silver nanoparticles, Lorenz-Mie scattering theory [8] was

The observed halfwidth of the plasmon resonance band at 415 nm equals to 100 nm that corresponds to the calculated average particle size of about 2.5 nm, which leads to the resonance wavelength of 407 nm. However, the SPR peak position of the parent glass heated up to the glass transition temperature is 415 nm (**Figure 4(b)**). This should mean that at this stage, NP is already in the shell.

Our simulation with silver bromide shell shows that several initial steps in the redshift cannot be achieved with this kind of crystal. A contrast in the refractive index between the silver bromide and the glass matrix is too high. Thus, we assumed that the refractive index of the shell is less than that of AgBr crystal. On the other hand, initial SPR peak position (415 nm) can be modeled with 2.42 nm shell thickness of pure NaBr crystal. However, refractive index contrast between the NaBr crystal and PTR glass does not allow to achieve redshift of the SPR to the 460 nm. Considering these facts, we suppose that the shell should be of mixed composition. Whereby composition of the shell can be either a solid solution of NaBr and AgBr crystals or a glass-crystal composite where AgBr inclusions present in the glass matrix. These assumptions lead to a shell with a refractive index

TEM image of the three-layer system (**Figure 5(a)**) provides us the outer diameter of the system, which can be roughly estimated to be 10 nm. This image was obtained from a sample with 460 nm SPR peak position. Thus, we conclude that 10 nm system diameter corresponds to the case where NaF is a host for a coreshell structure. Such a condition can be met only if the NaF layer thickness is above 3.17 nm. Considering the NP diameter of 2.5 nm, we adjudge that shell thickness equals to 0.57 nm (**Figure 5(b)**). It is a special mention that shell thickness correlates to the lattice constant of AgBr (0.597 nm) and NaBr (0.578 nm) crystals [23]. Since 0.57 nm is finite shell thickness, which already corresponds to the crystal lattice size, we will assume that the shell thickness is constant during the heat treatment procedure with the growing refractive index due to an increase in the AgBr

concentration either in the solid solution or in the crystal-glass mixture.

a decrease in the refractive index of the core-shell surrounding.

in the range of 1.68–2.25 depending on the AgBr fraction.

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

the silver nanoparticle.

used.

#### *Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

spectrum. Some processes occurring in this temperature range manifest themselves mostly in the UV. Such behavior had been discussed a lot in [18, 24] and attributed to the cerium and antimony ions in different valency. Further heating the sample leads to the formation of silver molecular clusters, thus causing an absorption increase in the visible. After it, when a temperature close to Tg is reached, it is seen that the SPR absorption peak corresponded to silver NPs manifesting in around 400 nm is already formed.

When starting the procedure of heat treating the sample, one can observe a significant redshift of the SPR peak location (**Figure 6**). Such behavior of the SPR band can be explained by the occurrence of the shell with a high refractive index on the silver nanoparticle.

During the photothermoinduced crystallization process of silver nanoparticles, precipitation occurs first, which is followed by formation of highly refractivity phase (silver bromide) when compared with glass matrix (1.498) [21]. It should be noted that after SPR peak location reaches 460 nm, it starts moving toward the shorter wavelengths up to 450 nm, and there, it remains independently from treatment duration time increase. The blueshift indicates the precipitation of NaF crystals that reduce the refractive index of the core-shell system surrounding. Stabilization of the peak location is due to the fact that NaF crystals already act as hosts for the core-shell structures. In the case of PTR glass without fluorine subjected to the same heat treatment procedure, the only redshift of the SPR resonance peak is observed (fluorinefree glass, **Figure 6(b)**). Hence, it can be concluded that the blue shift is inflicted by a decrease in the refractive index of the core-shell surrounding.

To estimate the size of silver nanoparticles, Lorenz-Mie scattering theory [8] was used.

The observed halfwidth of the plasmon resonance band at 415 nm equals to 100 nm that corresponds to the calculated average particle size of about 2.5 nm, which leads to the resonance wavelength of 407 nm. However, the SPR peak position of the parent glass heated up to the glass transition temperature is 415 nm (**Figure 4(b)**). This should mean that at this stage, NP is already in the shell.

Our simulation with silver bromide shell shows that several initial steps in the redshift cannot be achieved with this kind of crystal. A contrast in the refractive index between the silver bromide and the glass matrix is too high. Thus, we assumed that the refractive index of the shell is less than that of AgBr crystal.

On the other hand, initial SPR peak position (415 nm) can be modeled with 2.42 nm shell thickness of pure NaBr crystal. However, refractive index contrast between the NaBr crystal and PTR glass does not allow to achieve redshift of the SPR to the 460 nm. Considering these facts, we suppose that the shell should be of mixed composition. Whereby composition of the shell can be either a solid solution of NaBr and AgBr crystals or a glass-crystal composite where AgBr inclusions present in the glass matrix. These assumptions lead to a shell with a refractive index in the range of 1.68–2.25 depending on the AgBr fraction.

TEM image of the three-layer system (**Figure 5(a)**) provides us the outer diameter of the system, which can be roughly estimated to be 10 nm. This image was obtained from a sample with 460 nm SPR peak position. Thus, we conclude that 10 nm system diameter corresponds to the case where NaF is a host for a coreshell structure. Such a condition can be met only if the NaF layer thickness is above 3.17 nm. Considering the NP diameter of 2.5 nm, we adjudge that shell thickness equals to 0.57 nm (**Figure 5(b)**). It is a special mention that shell thickness correlates to the lattice constant of AgBr (0.597 nm) and NaBr (0.578 nm) crystals [23].

Since 0.57 nm is finite shell thickness, which already corresponds to the crystal lattice size, we will assume that the shell thickness is constant during the heat treatment procedure with the growing refractive index due to an increase in the AgBr concentration either in the solid solution or in the crystal-glass mixture.

*Holographic Materials and Applications*

*in the holographic grating with exposure 0.5 and 16 J/cm2*

*with the lowest possible concentration of fluorine.*

**Figure 3.**

**Figure 4.**

In [23], such effect was modeled considering the saturation of the RIC on exposure. Such saturation would lead to broadening of the grating fringes and decreasing the grating contrast. This situation can occur than in the interference pattern, maximum exposure already reached its saturation and thus does not contribute to further increase RIC. In this case, further exposure leads to a decrease in the contrast between the dark and the bright regions in terms of refractive index. However, our experiments show that fringes in the gratings with big exposure are narrower than that in gratings with small exposures **Figure 3(b)**. We suppose that the nonsinusoidal profile is a complex problem and cannot be explained with the saturation of exposure.

*(a) Absorption spectrum dynamic of the UV irradiated and heated up to 485°C for 90 min fluoride PTR glass. (b) Dynamics of SPR peak location depending on the heat treatment duration for the parent PTR glass and glass* 

*(a) Dependence of the RIC distribution between harmonics on dosage; (b) comparison of refractive index profile* 

*.*

**4. Fine structure of core-shell system inside fluoride PTR glass**

It is clear that (i) during photothermoinduced (PTI) crystallization process, the silver nanoparticles (NP) precipitate first (ii) that initiate sodium fluoride nanocrystals growth that is responsible for the refractive index changes. At the same time, the period between silver nanoparticles precipitation and sodium fluoride

In **Figure 4(a)**, the evolution of the UV-irradiated fluoride PTR glass spectrum in the course of the heating process (heating up to 485°C for 90 min) is shown. It is seen clearly that until 300°C, no significant changes appear in the visible absorption

**8**

crystals growth is still unclear.

#### **Figure 5.**

*(a) TEM image of a grating fringe in conventional PTR glass and (b) three-layer system model according to our estimations and the TEM image of a real system.*

**Figure 6.** *Estimated refractive index of the shell for solid solution (left) and Bruggeman (right) models.*

The first approach implies that the shell of our three-layer system is a solid solution of AgBr and NaBr crystal where during the heat treatment procedure, concentration of the AgBr increases. Refractive index of this system was calculated according to Varotsos [25] and its variation can be seen from **Figure 6(a)**.

It can be seen from **Figure 6(a)** that initial SPR peak position at 415 nm corresponds to 1.8% AgBr concentration in the solid solution. Thus, according to this model, initial position of the SPR equals to almost pure NaBr shell (with respect to the accuracy of the measurement and deconvolution). On the other hand, the final peak position equals to the 88.7% of the AgBr in the solid solution; therefore, at the end of this process according to this model, shell still represents a solid solution of the two crystals.

According to the second approach, a constant layer of the glass around the silver NP is filling up with AgBr crystal cells during the heat treatment. Since the refractive index of AgBr is higher than that of the surrounding glass, this process leads to the effective refractive index increase of the layer. Refractive index of this layer can be estimated with Bruggeman's two-component effective model [26]. According to our calculation, SPR peak position of 415 nm corresponds to 25% of AgBr inclusion volume fraction (**Figure 6(b)**); on the other hand, that of 460 nm corresponds to 89% of inclusion. Thus, one can see that precipitation speed of AgBr on the NP decreases with the crystal concentration growth. Accepting this model leads to conclusions that (i) the precipitation of AgBr is the most rapid at the beginning of PTI crystallization process and (ii) there is a probability that, in the course of the NP formation, the silver bromide can already be present nearby.

**11**

(>100 cm<sup>−</sup><sup>1</sup>

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

If we plot the concentration of AgBr in the shell as a function of the heat treatment duration for both above cases, the plots behave monotonically (**Figure 7**) and can be approximated with sigmoidal Weibull function with zero error. This means that both models demonstrate similar behavior with rapid growth at the beginning and severe speed decrease as the system is reaching its saturation (AgBr filling fac-

*Evolution of Ag mole fraction and AgBr filling factor during the heat treatment process. The corresponding SPR* 

In comparison with fluoride PTR glass, the chloride ones provide positive refractive index change, very high induced absorption in visible spectral range, and small size of the crystalline phase that suppresses scattering and allows recording holograms with high spatial resolution. However, the diffraction efficiency of such amplitude-phase holograms is limited according to the coupled wave theory [27]. Therefore, absorption of sinusoidal VBGs is increased and causes a decline of diffraction efficiency [28]. Thus, high absorption in near IR and visible spectral range hinders this advantage and it is highly desirable to suppress absorption of such VBGs and leave only phase grating. Since the modulation of the absorption in the grating fringes originates from the strong surface plasmon resonance of silver nanoparticles (AgNPs), which can be reduced by bleaching (photodestruction

**Figure 8** shows the optical density spectra of chloride PTR glass with recorded VBG before and after full femtosecond laser bleaching of the sample. It is seen that before the bleaching, glass possesses high absorption of surface plasmon resonance

shows the optical density spectrum of VBG after 10 hours of thermal treatment at 546°C when AgNPs concentration is fairly low. A dramatic decrease of optical

Two crystalline phases are precipitated in the volume of chloride PTR glass. The X-ray diffraction (XRD) analysis showed that the first phase is AgNPs, while the second phase is Na0.9Ag0.1Cl (NaCl-AgCl) nanocrystals. The XRD study was performed before and after laser bleaching. The XRD analysis demonstrated the photodestruction of AgNPs playing a role of the core on silver molecular clusters and ions and preservation of NaCl-AgCl nanocrystals in the form of a shell.

density in visible can be observed after laser bleaching of the sample.

) at 450 nm wavelength and visible spectral range. Inset to **Figure 9**

**5. Bleaching volume Bragg gratings in chloride PTR glass**

process) of the AgNPs using femtosecond laser radiation.

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

tor or mole fraction).

*peak positions are denoted in nm.*

**Figure 7.**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

#### **Figure 7.**

*Holographic Materials and Applications*

*estimations and the TEM image of a real system.*

**Figure 5.**

**Figure 6.**

The first approach implies that the shell of our three-layer system is a solid solution of AgBr and NaBr crystal where during the heat treatment procedure, concentration of the AgBr increases. Refractive index of this system was calculated

*(a) TEM image of a grating fringe in conventional PTR glass and (b) three-layer system model according to our* 

It can be seen from **Figure 6(a)** that initial SPR peak position at 415 nm corresponds to 1.8% AgBr concentration in the solid solution. Thus, according to this model, initial position of the SPR equals to almost pure NaBr shell (with respect to the accuracy of the measurement and deconvolution). On the other hand, the final peak position equals to the 88.7% of the AgBr in the solid solution; therefore, at the end of this process accord-

According to the second approach, a constant layer of the glass around the silver NP is filling up with AgBr crystal cells during the heat treatment. Since the refractive index of AgBr is higher than that of the surrounding glass, this process leads to the effective refractive index increase of the layer. Refractive index of this layer can be estimated with Bruggeman's two-component effective model [26]. According to our calculation, SPR peak position of 415 nm corresponds to 25% of AgBr inclusion volume fraction (**Figure 6(b)**); on the other hand, that of 460 nm corresponds to 89% of inclusion. Thus, one can see that precipitation speed of AgBr on the NP decreases with the crystal concentration growth. Accepting this model leads to conclusions that (i) the precipitation of AgBr is the most rapid at the beginning of PTI crystallization process and (ii) there is a probability that, in the course of the

according to Varotsos [25] and its variation can be seen from **Figure 6(a)**.

*Estimated refractive index of the shell for solid solution (left) and Bruggeman (right) models.*

ing to this model, shell still represents a solid solution of the two crystals.

NP formation, the silver bromide can already be present nearby.

**10**

*Evolution of Ag mole fraction and AgBr filling factor during the heat treatment process. The corresponding SPR peak positions are denoted in nm.*

If we plot the concentration of AgBr in the shell as a function of the heat treatment duration for both above cases, the plots behave monotonically (**Figure 7**) and can be approximated with sigmoidal Weibull function with zero error. This means that both models demonstrate similar behavior with rapid growth at the beginning and severe speed decrease as the system is reaching its saturation (AgBr filling factor or mole fraction).

#### **5. Bleaching volume Bragg gratings in chloride PTR glass**

In comparison with fluoride PTR glass, the chloride ones provide positive refractive index change, very high induced absorption in visible spectral range, and small size of the crystalline phase that suppresses scattering and allows recording holograms with high spatial resolution. However, the diffraction efficiency of such amplitude-phase holograms is limited according to the coupled wave theory [27]. Therefore, absorption of sinusoidal VBGs is increased and causes a decline of diffraction efficiency [28]. Thus, high absorption in near IR and visible spectral range hinders this advantage and it is highly desirable to suppress absorption of such VBGs and leave only phase grating. Since the modulation of the absorption in the grating fringes originates from the strong surface plasmon resonance of silver nanoparticles (AgNPs), which can be reduced by bleaching (photodestruction process) of the AgNPs using femtosecond laser radiation.

**Figure 8** shows the optical density spectra of chloride PTR glass with recorded VBG before and after full femtosecond laser bleaching of the sample. It is seen that before the bleaching, glass possesses high absorption of surface plasmon resonance (>100 cm<sup>−</sup><sup>1</sup> ) at 450 nm wavelength and visible spectral range. Inset to **Figure 9** shows the optical density spectrum of VBG after 10 hours of thermal treatment at 546°C when AgNPs concentration is fairly low. A dramatic decrease of optical density in visible can be observed after laser bleaching of the sample.

Two crystalline phases are precipitated in the volume of chloride PTR glass. The X-ray diffraction (XRD) analysis showed that the first phase is AgNPs, while the second phase is Na0.9Ag0.1Cl (NaCl-AgCl) nanocrystals. The XRD study was performed before and after laser bleaching. The XRD analysis demonstrated the photodestruction of AgNPs playing a role of the core on silver molecular clusters and ions and preservation of NaCl-AgCl nanocrystals in the form of a shell.

#### **Figure 8.**

*Absorption spectra of as-prepared PTR glass, VBGs before and after bleaching. Inset: optical density spectrum of VBG with small AgNPs concentration.*

**Figure 9.** *The mechanism for bleaching of AgNPs hologram.*

The process of photodestruction of metallic nanoparticles is well known for different types of glass [29, 30]. The absorption of ultrashort pulse energy leads to local heating and the explosion of the metallic particle [31]. In **Figure 9**, process of AgNPs surrounded by NaCl-AgCl shell, destruction, is shown schematically [32]. The femtosecond bleaching transforms AgNPs into silver clusters. However, due to the presence of shell, the particle fragments cannot move far from the origin as available space is limited by the interior volume of NaCl-AgBr crystal. It should be noted that nanoparticle restoration time under thermal treatment takes several minutes that also indicates that silver clusters close to each other after bleaching.

Considering that the absorption caused by AgNPs plasmon resonance is not present at the spectrum of the grating, it was assumed that the grating becomes purely phase type after laser processing. Previously, it was shown that VBGs in chloride PTR glass are of a mixed type, that is, both refractive and absorption indexes are modulated in the glass volume [33]. In accordance with the coupled wave theory, the coupling constant (χ) responsible for the transfer of energy between the reference (R) and signal (S) wave contains refractive index modulation amplitude (RIMA) and absorption index modulation amplitude (AIMA). Hence, removal of the one causes a natural decline of the coupling efficiency as described by Collier [34].

**13**

**Figure 11.**

range of 1–12 J/cm<sup>2</sup>

**Figure 10.**

*AgNPs in VBGs.*

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

In order to characterize the contribution of RIMA and AIMA to the VBGs performance, one should analyze angular selectivity contour of zero diffraction order. It is critical as the first-order diffraction selectivity contour looks similar for both

*Experimental and fitted curves of angular selectivity contours before (a) and after (b) photodestruction of* 

Experimental and fitted angular selectivity contours for the holograms recorded in chloride PTR glass are shown in **Figure 10**. One can notice an asymmetric shape of the zeroth diffraction order contour before laser bleaching with respect to Bragg angle. It directly points out at mixed nature of the grating [28, 33, 35]. After laser processing, the contour becomes symmetric as shown in **Figure 10(b)**. These factors indicate the lack of absorption modulation in the hologram, so it became purely phase. Hence, the

amplitude-phase and phase gratings with the same coupling.

modification of the contour shape indicates a weakening of the coupling.

As it is shown in **Figure 11(a)** for the exposures above 6 J/cm2

The further analysis consists of the estimation of the AIMA and RIMA [27, 28, 33, 35]. The calculation of these parameters can be carried out by the fitting of the contours obtained in the experiment with those predicted by the theory (**Figure 11**, red curves). The performed fitting of the experimental results allows one to evaluate RIMA magnitude for each grating in the exposure

before and after bleaching (**Figure 11(a)**).

of the crystalline phase and concentration of AgNPs saturated because RIMA value stopped increasing. Thus, all available silver and chlorine ions in the hologram fringe were utilized. Moreover, it should be noted that the RIMA values decreased after laser bleaching for all exposures and the magnitude of the change depends on the exposure.

*(a) RIMA before and after laser bleaching of VBGs. (b) Difference of RIMA before and after laser bleaching* 

*(curve with squares) and AIMA before laser bleaching (curve with circles).*

, the volume fraction

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

**Figure 10.**

*Holographic Materials and Applications*

**12**

**Figure 9.**

**Figure 8.**

*The mechanism for bleaching of AgNPs hologram.*

*of VBG with small AgNPs concentration.*

The process of photodestruction of metallic nanoparticles is well known for different types of glass [29, 30]. The absorption of ultrashort pulse energy leads to local heating and the explosion of the metallic particle [31]. In **Figure 9**, process of AgNPs surrounded by NaCl-AgCl shell, destruction, is shown schematically [32]. The femtosecond bleaching transforms AgNPs into silver clusters. However, due to the presence of shell, the particle fragments cannot move far from the origin as available space is limited by the interior volume of NaCl-AgBr crystal. It should be noted that nanoparticle restoration time under thermal treatment takes several minutes that also indicates that silver clusters close to each other after bleaching. Considering that the absorption caused by AgNPs plasmon resonance is not present at the spectrum of the grating, it was assumed that the grating becomes purely phase type after laser processing. Previously, it was shown that VBGs in chloride PTR glass are of a mixed type, that is, both refractive and absorption indexes are modulated in the glass volume [33]. In accordance with the coupled wave theory, the coupling constant (χ) responsible for the transfer of energy between the reference (R) and signal (S) wave contains refractive index modulation amplitude (RIMA) and absorption index modulation amplitude (AIMA). Hence, removal of the one causes a

*Absorption spectra of as-prepared PTR glass, VBGs before and after bleaching. Inset: optical density spectrum* 

natural decline of the coupling efficiency as described by Collier [34].

*Experimental and fitted curves of angular selectivity contours before (a) and after (b) photodestruction of AgNPs in VBGs.*

In order to characterize the contribution of RIMA and AIMA to the VBGs performance, one should analyze angular selectivity contour of zero diffraction order. It is critical as the first-order diffraction selectivity contour looks similar for both amplitude-phase and phase gratings with the same coupling.

Experimental and fitted angular selectivity contours for the holograms recorded in chloride PTR glass are shown in **Figure 10**. One can notice an asymmetric shape of the zeroth diffraction order contour before laser bleaching with respect to Bragg angle. It directly points out at mixed nature of the grating [28, 33, 35]. After laser processing, the contour becomes symmetric as shown in **Figure 10(b)**. These factors indicate the lack of absorption modulation in the hologram, so it became purely phase. Hence, the modification of the contour shape indicates a weakening of the coupling.

The further analysis consists of the estimation of the AIMA and RIMA [27, 28, 33, 35]. The calculation of these parameters can be carried out by the fitting of the contours obtained in the experiment with those predicted by the theory (**Figure 11**, red curves). The performed fitting of the experimental results allows one to evaluate RIMA magnitude for each grating in the exposure range of 1–12 J/cm<sup>2</sup> before and after bleaching (**Figure 11(a)**).

As it is shown in **Figure 11(a)** for the exposures above 6 J/cm2 , the volume fraction of the crystalline phase and concentration of AgNPs saturated because RIMA value stopped increasing. Thus, all available silver and chlorine ions in the hologram fringe were utilized. Moreover, it should be noted that the RIMA values decreased after laser bleaching for all exposures and the magnitude of the change depends on the exposure.

#### **Figure 11.**

*(a) RIMA before and after laser bleaching of VBGs. (b) Difference of RIMA before and after laser bleaching (curve with squares) and AIMA before laser bleaching (curve with circles).*

The maximum value of RIMA was reduced from 8.6 × 10<sup>−</sup><sup>4</sup> down to 5.1 × 10<sup>−</sup><sup>4</sup> , that is, it became nearly two times lower after AgNPs photodestruction. As it is following from **Figure 11(b)**, the difference in the RIMA before and after laser processing and the AIMA vs. exposure demonstrates identical dependence on the exposure. It illustrates that additional RIMA, which was lost during the laser bleaching, was caused by the presence of the AgNPs in the hologram fringes and the value is correlated with their concentration. Although the dip of the RIMA value at the maximum is considerable, it was still sufficient to increase diffraction efficiency from 25 to 86%.

Thus, ultrashort laser bleaching of chloride PTR glass with VBGs causes a decline of RIMA from 8.6 × 10<sup>−</sup><sup>4</sup> to 5.1 × 10<sup>−</sup><sup>4</sup> , which is still adequate for efficient VBGs operation. After the laser processing, the hologram becomes purely phase modulated, so it is capable to reach high diffraction efficiency at specific exposure, thermal treatment, and appropriate grating thickness. The transmission range of the VBGs expanded dramatically in visible spectral range opening avenues toward using them for high-power laser applications. Additionally, the smaller size of nanocrystals coupled with the absence of absorption in the visible region boosts the performance of such holograms at the smaller grating periods.

#### **6. Thermal stability of volume Bragg gratings in chloride PTR glass**

As-prepared chloride PTR glass is transparent in whole visible and near IR spectral range until 2.5 μm. The main drawback of chloride PTR glass is absorption in visible and near IR spectral range, caused by AgNPs possessing surface plasmon resonance (SPR) at 410–450 nm. Chloride PTR glass has high SPR absorption (>100 cm<sup>−</sup><sup>1</sup> ) with considerable tail at near IR. In Section 5, we demonstrated that AgNPs absorption can be reduced using femtosecond laser radiation. Moreover, it was also demonstrated that AgNPs in chloride PTR glass can be bleached without photodestruction of NaCl-AgCl nanocrystals, responsible for refractive index modulation in VBGs. In this chapter, the bleaching effect on thermal stability of gratings recorded on chloride PTR glass is discussed. Such bleaching of chloride PTR glass with VBGs can open new opportunities for high-power laser diode (LD) applications in near-IR spectral range.

Absorption of gratings in chloride PTR glass at the wavelength of LD (972 nm) is 1.8 cm<sup>−</sup><sup>1</sup> before laser bleaching and 0.05 cm<sup>−</sup><sup>1</sup> after the bleaching. The latter corresponds to the absorption of as-prepared glass. It is clear that higher absorption of LD pumping light at wavelength 972 nm causes heat accumulation at VBG. Thus, at some point, the input power cannot be dissipated with a low thermal conductivity of the glass and the local temperature starts increasing. As following, the parameters of VBG such as refractive index difference, grating period, and absorption may change significantly.

For studying the thermal stability of VBGs, we measured the angular selectivity contour of transmission VBG at a wavelength of He-Ne laser before and after bleaching (**Figure 12**). Radiation of CW laser diode at wavelength 972 nm was used as a hitting source.

**Figure 12(a)** shows angular selectivity curves of VBG under different pumping of LD before femtosecond laser bleaching. A shift of Bragg wavelength under different pumping power indicates the increase of the grating period, which directly effects Bragg conditions for diffracted He-Ne laser beam. It is worth noting that the shape of the curve without laser diode irradiation is complex and contains two peaks, which indicates that the grating is overmodulated. The amplitude of central peak gradually decreases with the growth of the laser diode intensity. The actual amplitude drop is even more pronounced if one subtracts the background radiation, which can be observed far from the Bragg angle. So, it is reasonable to conclude that under high

**15**

**Figure 13.**

cm2

**Figure 12.**

from the following equation:

*ΔT = \_\_\_\_ Δd*

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

power irradiation, the diffraction efficiency drops approximately by 30%. This result can be understood by taking into account the fact that at the beam intensity of 40 W/

*Angular selectivity at first order of diffraction of initial (a) and bleached (b) hologram.*

, the grating temperature is about 350°C. It is known that the melting temperature of metal particle decreases for smaller particles. For AgNPs in silicate glass, melting temperature can be lower than 150°C, and optical properties of composite materials with embedded AgNPs change dramatically. The plasmon resonance absorption band of a liquid NP is broadening, which affects RIMA and AIMA values as it was discussed in the previous section. Melted AgNPs get crystallized back after cooling of the glass substrate with VBG and optical properties of the grating return to initial values.

**Figure 12(b)** shows angular selectivity contours after laser bleaching of VBGs under the different intensity of the LD diode beam. One can notice that the shift of the Bragg angle became smaller and the amplitude of the peak states unchanged even at higher fluence. The temperature increase in the irradiated area can be found

where d is a grating period, n is a refractive index of glass, and a is a coefficient of thermal expansion. A grating temperature increase after laser bleaching at the level of 40°C for the same laser intensity is almost nine times smaller than before laser bleaching. **Figure 13** shows temperature increase and correspondent Bragg wavelength shift

*Dependence of temperature change and Bragg wavelength on the power density of the pumping source.*

<sup>d</sup> <sup>∙</sup> <sup>a</sup> (2)

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

**Figure 12.** *Angular selectivity at first order of diffraction of initial (a) and bleached (b) hologram.*

power irradiation, the diffraction efficiency drops approximately by 30%. This result can be understood by taking into account the fact that at the beam intensity of 40 W/ cm2 , the grating temperature is about 350°C. It is known that the melting temperature of metal particle decreases for smaller particles. For AgNPs in silicate glass, melting temperature can be lower than 150°C, and optical properties of composite materials with embedded AgNPs change dramatically. The plasmon resonance absorption band of a liquid NP is broadening, which affects RIMA and AIMA values as it was discussed in the previous section. Melted AgNPs get crystallized back after cooling of the glass substrate with VBG and optical properties of the grating return to initial values.

**Figure 12(b)** shows angular selectivity contours after laser bleaching of VBGs under the different intensity of the LD diode beam. One can notice that the shift of the Bragg angle became smaller and the amplitude of the peak states unchanged even at higher fluence. The temperature increase in the irradiated area can be found from the following equation:

$$
\Delta T = \frac{\Delta d}{\mathbf{d} \cdot \mathbf{a}} \tag{2}
$$

where d is a grating period, n is a refractive index of glass, and a is a coefficient of thermal expansion. A grating temperature increase after laser bleaching at the level of 40°C for the same laser intensity is almost nine times smaller than before laser bleaching. **Figure 13** shows temperature increase and correspondent Bragg wavelength shift

**Figure 13.** *Dependence of temperature change and Bragg wavelength on the power density of the pumping source.*

*Holographic Materials and Applications*

decline of RIMA from 8.6 × 10<sup>−</sup><sup>4</sup>

The maximum value of RIMA was reduced from 8.6 × 10<sup>−</sup><sup>4</sup>

is, it became nearly two times lower after AgNPs photodestruction. As it is following from **Figure 11(b)**, the difference in the RIMA before and after laser processing and the AIMA vs. exposure demonstrates identical dependence on the exposure. It illustrates that additional RIMA, which was lost during the laser bleaching, was caused by the presence of the AgNPs in the hologram fringes and the value is correlated with their concentration. Although the dip of the RIMA value at the maximum is consider-

able, it was still sufficient to increase diffraction efficiency from 25 to 86%. Thus, ultrashort laser bleaching of chloride PTR glass with VBGs causes a

performance of such holograms at the smaller grating periods.

before laser bleaching and 0.05 cm<sup>−</sup><sup>1</sup>

to 5.1 × 10<sup>−</sup><sup>4</sup>

**6. Thermal stability of volume Bragg gratings in chloride PTR glass**

As-prepared chloride PTR glass is transparent in whole visible and near IR spectral range until 2.5 μm. The main drawback of chloride PTR glass is absorption in visible and near IR spectral range, caused by AgNPs possessing surface plasmon resonance (SPR) at 410–450 nm. Chloride PTR glass has high SPR absorption (>100 cm<sup>−</sup><sup>1</sup>

considerable tail at near IR. In Section 5, we demonstrated that AgNPs absorption can be reduced using femtosecond laser radiation. Moreover, it was also demonstrated that AgNPs in chloride PTR glass can be bleached without photodestruction of NaCl-AgCl nanocrystals, responsible for refractive index modulation in VBGs. In this chapter, the bleaching effect on thermal stability of gratings recorded on chloride PTR glass is discussed. Such bleaching of chloride PTR glass with VBGs can open new opportunities for high-power laser diode (LD) applications in near-IR spectral range. Absorption of gratings in chloride PTR glass at the wavelength of LD (972 nm)

responds to the absorption of as-prepared glass. It is clear that higher absorption of LD pumping light at wavelength 972 nm causes heat accumulation at VBG. Thus, at some point, the input power cannot be dissipated with a low thermal conductivity of the glass and the local temperature starts increasing. As following, the parameters of VBG such as refractive index difference, grating period, and absorption

For studying the thermal stability of VBGs, we measured the angular selectivity contour of transmission VBG at a wavelength of He-Ne laser before and after bleaching (**Figure 12**). Radiation of CW laser diode at wavelength 972 nm was used

**Figure 12(a)** shows angular selectivity curves of VBG under different pumping of LD before femtosecond laser bleaching. A shift of Bragg wavelength under different pumping power indicates the increase of the grating period, which directly effects Bragg conditions for diffracted He-Ne laser beam. It is worth noting that the shape of the curve without laser diode irradiation is complex and contains two peaks, which indicates that the grating is overmodulated. The amplitude of central peak gradually decreases with the growth of the laser diode intensity. The actual amplitude drop is even more pronounced if one subtracts the background radiation, which can be observed far from the Bragg angle. So, it is reasonable to conclude that under high

VBGs operation. After the laser processing, the hologram becomes purely phase modulated, so it is capable to reach high diffraction efficiency at specific exposure, thermal treatment, and appropriate grating thickness. The transmission range of the VBGs expanded dramatically in visible spectral range opening avenues toward using them for high-power laser applications. Additionally, the smaller size of nanocrystals coupled with the absence of absorption in the visible region boosts the

down to 5.1 × 10<sup>−</sup><sup>4</sup>

, which is still adequate for efficient

after the bleaching. The latter cor-

, that

) with

**14**

is 1.8 cm<sup>−</sup><sup>1</sup>

may change significantly.

as a hitting source.

as functions of pump irradiance for VBGs before and after laser bleaching. A dramatic decrease in temperature after laser bleaching made it possible to increase pumping intensity almost four times from 40 to 145 W/cm<sup>2</sup> . In our experimental conditions, strong temperature gradients in the irradiated area resulted in the glass cracking when the pump intensity exceeds 145 W/cm2 .

Thus, the bleaching technology can dramatically decrease heating of the grating under high power laser radiation, which enhances the stability of the grating's parameters such as period and RIMA.

#### **7. Conclusions**

Recent achievements of ITMO University (St. Petersburg, Russia) in the investigation of properties of holographic volume Bragg gratings in fluoride and chloride photothermorefractive glass are demonstrated. Some futures of the holograms are highlighted and discussed. Some ways of improvement of characteristics of the holograms are suggested.

Namely, the mechanism of refractive index change in fluoride photothermorefractive glass during photothermoinduced crystallization and refractive index profile of the volume Bragg gratings were discussed. It is shown that refractive index change in the exposed area appears primarily due to the NaF nanocrystal inclusions in the glass matrix, which according to the Maxwell-Garnet theory decreases the effective refractive index of such composite. It is shown experimentally that residual stresses have no effect on the refractive index change. Estimated refractive index difference between the composite and unperturbed glass is in a good agreement with the refractive index change obtained by the grating analysis.

The refractive index profile in the volume Bragg gratings on fluoride photothermorefractive glass was studied. It was shown that the nonsinusoidal profile of the gratings at high exposures cannot be explained by saturation of exposure since the fraction of the first harmonic stays the same at exposures above 6 J/cm2 . In addition, it was shown that at higher exposures, grating fringes became narrower than that of the gratings with close to sinusoidal profile.

A fine structure of the core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell was explored. In particular, for the three-layer system consisting of a silver nanoparticle, the bromide shell, and sodium fluoride shell, we made a numerical analysis of the peak location of surface plasmon resonance. The analysis showed that the thickness of the high refractive index shell does not exceed 0.5–0.6 nm. This result is in a good correlation with the unit cell size of the bromide crystals. We showed that the initial peak location of surface plasmon resonance cannot be explained by the formation of pure silver bromide shell or by the size of the silver nanoparticle. Moreover, the final peak location cannot be explained by the formation of sodium bromide shell alone. We demonstrated that the observed short-wavelength shift of the surface plasmon resonance peak at the later stage of photothermoinduced crystallization is due to the precipitation of sodium fluoride nanocrystals.

The optical properties of volume Bragg gratings in chloride PTR glass after femtosecond laser bleaching have been studied. The bleaching procedure significantly reduces the absorption of volume Bragg gratings (from 100 to 0.1 cm<sup>−</sup><sup>1</sup> ). Laser bleaching does not affect the amplitude of refractive index modulation of the grating. Femtosecond laser radiation breaks down silver nanoparticle to silver clusters and ions, but the NaCl-AgCl shell remains almost the same. Thus, the amplitude-phase grating is modified in the pure phase grating by femtosecond laser photodestruction of silver nanoparticles in the volume of PTR glass. Thus, the

**17**

provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Nikonorov Nikolay\*, Ivanov Sergei, Dubrovin Victor and Klyukin Dmitry Saint-Petersburg National Research University of Information Technologies,

Mechanics and Optics, St. Petersburg, Russian Federation

\*Address all correspondence to: nikonorov@oi.ifmo.ru

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

nanoparticles opens new opportunities for high laser power application.

increase of glass transmission in visible spectral range after laser bleaching of silver

It was shown that the bleaching procedure increases the thermal stability of the volume Bragg gratings under near IR laser diode pumping. So, the temperature change

from 350 to 38°C. The bleaching of volume gratings allowed us to reduce temperature

This work was supported by the Ministry of Science and Higher Education of

can dramatically decrease heating of the grating under high power laser radiation, which enhances the stability of the grating parameters such as period and amplitude of refractive index modulation. It should be pointed out that the problem of silver nanoparticles absorption hinders not only chloride photothermorefractive glass because so far, all types of PTR glass (fluoride and bromide) also make use of silver nanoparticles precipitation for photothermoinduced crystallization. Thus, we believe that this approach can be utilized for bleaching of other types of PTR glass as well.

power density has reduced

). Thus, the bleaching technology

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

of the glass with volume Bragg grating under 40 W/cm2

drift of Bragg wavelength from 50 to 6 pm/(W/cm2

Russian Federation (Project 16.1651.2017/4.6).

**Acknowledgements**

**Author details**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

increase of glass transmission in visible spectral range after laser bleaching of silver nanoparticles opens new opportunities for high laser power application.

It was shown that the bleaching procedure increases the thermal stability of the volume Bragg gratings under near IR laser diode pumping. So, the temperature change of the glass with volume Bragg grating under 40 W/cm2 power density has reduced from 350 to 38°C. The bleaching of volume gratings allowed us to reduce temperature drift of Bragg wavelength from 50 to 6 pm/(W/cm2 ). Thus, the bleaching technology can dramatically decrease heating of the grating under high power laser radiation, which enhances the stability of the grating parameters such as period and amplitude of refractive index modulation. It should be pointed out that the problem of silver nanoparticles absorption hinders not only chloride photothermorefractive glass because so far, all types of PTR glass (fluoride and bromide) also make use of silver nanoparticles precipitation for photothermoinduced crystallization. Thus, we believe that this approach can be utilized for bleaching of other types of PTR glass as well.

#### **Acknowledgements**

*Holographic Materials and Applications*

the pump intensity exceeds 145 W/cm2

parameters such as period and RIMA.

**7. Conclusions**

holograms are suggested.

intensity almost four times from 40 to 145 W/cm<sup>2</sup>

as functions of pump irradiance for VBGs before and after laser bleaching. A dramatic decrease in temperature after laser bleaching made it possible to increase pumping

strong temperature gradients in the irradiated area resulted in the glass cracking when

Thus, the bleaching technology can dramatically decrease heating of the grating under high power laser radiation, which enhances the stability of the grating's

Recent achievements of ITMO University (St. Petersburg, Russia) in the investigation of properties of holographic volume Bragg gratings in fluoride and chloride photothermorefractive glass are demonstrated. Some futures of the holograms are highlighted and discussed. Some ways of improvement of characteristics of the

Namely, the mechanism of refractive index change in fluoride photothermorefractive glass during photothermoinduced crystallization and refractive index profile of the volume Bragg gratings were discussed. It is shown that refractive index change in the exposed area appears primarily due to the NaF nanocrystal inclusions in the glass matrix, which according to the Maxwell-Garnet theory decreases the effective refractive index of such composite. It is shown experimentally that residual stresses have no effect on the refractive index change. Estimated refractive index difference between the composite and unperturbed glass is in a good agree-

The refractive index profile in the volume Bragg gratings on fluoride photothermorefractive glass was studied. It was shown that the nonsinusoidal profile of the gratings at high exposures cannot be explained by saturation of exposure since the

it was shown that at higher exposures, grating fringes became narrower than that of

A fine structure of the core-shell system inside fluoride PTR glass in which a silver nanoparticle presents the core and crystalline phases of silver bromide and sodium fluoride present the shell was explored. In particular, for the three-layer system consisting of a silver nanoparticle, the bromide shell, and sodium fluoride shell, we made a numerical analysis of the peak location of surface plasmon resonance. The analysis showed that the thickness of the high refractive index shell does not exceed 0.5–0.6 nm. This result is in a good correlation with the unit cell size of the bromide crystals. We showed that the initial peak location of surface plasmon resonance cannot be explained by the formation of pure silver bromide shell or by the size of the silver nanoparticle. Moreover, the final peak location cannot be explained by the formation of sodium bromide shell alone. We demonstrated that the observed short-wavelength shift of the surface plasmon resonance peak at the later stage of photothermoinduced

ment with the refractive index change obtained by the grating analysis.

fraction of the first harmonic stays the same at exposures above 6 J/cm2

crystallization is due to the precipitation of sodium fluoride nanocrystals.

The optical properties of volume Bragg gratings in chloride PTR glass after femtosecond laser bleaching have been studied. The bleaching procedure significantly reduces the absorption of volume Bragg gratings (from 100 to 0.1 cm<sup>−</sup><sup>1</sup>

Laser bleaching does not affect the amplitude of refractive index modulation of the grating. Femtosecond laser radiation breaks down silver nanoparticle to silver clusters and ions, but the NaCl-AgCl shell remains almost the same. Thus, the amplitude-phase grating is modified in the pure phase grating by femtosecond laser photodestruction of silver nanoparticles in the volume of PTR glass. Thus, the

the gratings with close to sinusoidal profile.

.

. In our experimental conditions,

. In addition,

).

**16**

This work was supported by the Ministry of Science and Higher Education of Russian Federation (Project 16.1651.2017/4.6).

#### **Author details**

Nikonorov Nikolay\*, Ivanov Sergei, Dubrovin Victor and Klyukin Dmitry Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, St. Petersburg, Russian Federation

\*Address all correspondence to: nikonorov@oi.ifmo.ru

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Stookey SD. Photosensetive glass. Industrial & Engineering Chemistry Research. 1949;**41**(4):856-861

[2] Stookey SD, Beall GH, Pierson JE. Full-color photosensitive glass. Journal of Applied Physics. 1978;**49**(10):5114

[3] Pierson JE, Stookey SD. United States Patent 4,057,408; 1977

[4] Pierson EJ, Stookey SD. United States Patent 4,017,318; 1977

[5] Panysheva EI, Tunimanova IV, Tsekhomskiĭ VA. A study of coloring in polychromatic glasses. Fizika i Khimiya Stekla. 1990;**16**(2):239-244. In Russian

[6] Dotsenko AV, Efimov AM, Zakharov VK, Panysheva EI, Tunimanova IV. On the absorption spectra of polychromatic. Fizika i Khimiya Stekla. 1985;**11**(5):592-595. In Russian

[7] Glebov LB, Nikonorov NV, Panysheva EI, Petrovskii GT, Savvin VV, Tunimanova IV, et al. New possibilities of photosensitive glasses for the recording of volume phase diagrams. Optika i Spektroskopiya. 1992;**73**(2):404-412. In Russian

[8] Kuchinskii SA, Nikonorov NV, Panysheva EI, Savvin VV, Tunimanova I. Properties of volume phase holograms on polychromatic glasses. Optika i Spektroskopiya. 1991;**70**(6):1286-1300. In Russian

[9] Nikonorov NV, Panysheva EI. Polychromatic glasses-a new medium for optical data recording. In: Opticheskoe Izobrazhenie i Registriruyushchie Sredy (All-Union Conference "Optical Image and Recording Media"). Leningrad: GOI; 1990. pp. 2-48. In Russian

[10] Glebov LB, Nikonorov NV, Panysheva EI, Tunimanova IV, Savvin VV, Tsekhomskii VA. Photothermorefractive

glass. In: IF AN Latv. SSR, editor. Trudy VII Vsesoyuznoi Konferentsii po Radiatsionnoi Fizike i Khimii Neorganicheskikh Materialov (Proceedings of VII All-Union Conference on Radiation Physics and Chemistry of Inorganic Materials); Riga. 1989. p. 527. In Russian

[11] Efimov OM, Glebov LB, Glebova LN, Richardson KC, Smirnov VI. High efficiency Bragg gratings in photothermo-refractive glass. Applied Optics. 1999;**38**(2):619-627

[12] Glebov LB, Glebova LN, Richardson KA, Smirnov VI. Photo-induced processes in photo-thermo-refractive glasses. In: Soc AC, editor. Proceedings of XV Congress on Glass. San Francisco; 1998

[13] Ivanov SA, Ignat'ev AI, Nikonorov NV, Aseev VA. Holographic characteristics of a modified photothermorefractive glass. Journal of Optical Technology. 2014;**81**(6):356-360

[14] Nikonorov NV, Panysheva EI, Tunimanova IV, Chukharev AV. Influence of glass composition on the refractive index change upon photothermo induced crystallization. Glass Physics and Chemistry. 2001;**27**(3):241-249. Available from: http://link.springer.com/article/10. 1023/A:1011392301107

[15] Glebova L, Lumeau J, Klimov M, Zanotto ED, Glebov LB. Role of bromine on the thermal and optical properties of photo-thermo-refractive glass. Journal of Non-Crystalline Solids. 2008;**354** (2-9):456-461. Available from: http:// linkinghub.elsevier.com/retrieve/pii/ S0022309307010903

[16] Glebov LB, Nikonorov NV, Panysheva EI, Petrovskii GT, Savvin VV, Tunimanova IV, et al. New ways to use photosensitive glasses for recording volume phase holograms. Optics and Spectroscopy. 1992;**73**(2):237-241

**19**

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass*

spectroscopic studies of photo-thermorefractive glasses. II. Manifestations of Ce3+ and Ce(IV) valence states in the UV absorption spectrum of cerium-doped photo-thermo-refractive matrix glasses. Journal of Non-Crystalline Solids.

[25] Varotsos P. Determination of the dielectric constant of alkali halide mixed crystals. Physica Status Solidi.

[26] Bruggeman VDAG. Berechnung verschiedener physikalischer Konstanten von heterogenen

Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen. Annalen der

[27] Kogelnik H. Coupled wave theory for thick hologram gratings. Bell System Technical Journal. 1969;**48**(9):2909-2947

[28] Carretero L, Madrigal RF, Fimia A, Blaya S, Beléndez A. Study of angular responses of mixed amplitude-phase holographic gratings: Shifted Borrmann effect. Optics Letters. 2001;**26**(11): 786-788. Available from: http://www. ncbi.nlm.nih.gov/pubmed/18040450

[29] Podlipensky AV, Grebenev V, Seifert G, Graener H. Ionization and photomodification of Ag nanoparticles in soda-lime glass by 150 fs laser irradiation:

A luminescence study. Journal of Luminescence. 2004;**109**:135-142

2000;**251**:181-203

[30] Bigot J, Halte V, Merle J, Daunois A. Electron dynamics in metallic nanoparticles. Chemical Physics.

[31] Hashimoto S, Werner D, Uwada T. Studies on the interaction of pulsed lasers with plasmonic gold nanoparticles

toward light manipulation, heat management, and nanofabrication. Journal of Photochemistry and Photobiology C Photochemistry Reviews. 2012;**13**(1):28-54

2013;**361**(1):26-37

1980;**100**(2):K133-K138

Physik. 1935;**416**(7):636-664

*DOI: http://dx.doi.org/10.5772/intechopen.85289*

[17] Dyamant I, Abyzov AS, Fokin VM, Zanotto ED, Lumeau J, Glebova LN, et al. Crystal nucleation and growth kinetics of NaF in photo-thermorefractive glass. Journal of Non-Crystalline Solids. 2013;**378**:115-120

[18] Efimov AM, Ignatiev AI, Nikonorov NV, Postnikov ES. Quantitative UV-VIS spectroscopic studies of photo-thermo-refractive glasses. I. Intrinsic, bromine-related, and impurity-related UV absorption in photo-thermo-refractive glass matrices. Journal of Non-Crystalline Solids. 2011;**357**(19-20):3500-3512

[19] Dubrovin VD, Ignatiev AI,

cfm?URI=ome-6-5-1701

2015;**33**(17):3730-3735

InTech; 2017. pp. 435-461

Nikonorov NV. Chloride photo-thermorefractive glasses. Optical Materials Express. 2016;**6**(5):1701. Available from: https://www.osapublishing.org/abstract.

[20] Sgibnev YM, Nikonorov NV, Vasilev VN, Ignatiev AI. Optical gradient waveguides in photo-thermo-refractive glass formed by ion exchange method. Journal of Lightwave Technology.

[21] Nikonorov N, Ivanov S, Dubrovin V,

refractive glasses for holographic optical elements: Properties and applications. In: Naydenova I, editor. Holographic Materials and Optical Systems. Dublin:

[22] Lumeau J, Glebova L, Golubkov V, Zanotto ED, Glebov LB. Origin of crystallization-induced refractive index changes in photo-thermo-refractive glass. Optical Materials. 2009;**32**(1):139-146

[23] Lumeau J, Glebov LB. Effect of the refractive index change kinetics of photosensitive materials on the diffraction efficiency of reflecting Bragg gratings. Applied Optics. 2013;**52**(17):3993-3997

[24] Efimov AM, Ignatiev AI, Nikonorov NV, Postnikov ES. Quantitative UV-VIS

Ignatiev A. New photo-thermo-

*Features of Volume Holograms in Fluoride and Chloride Photothermorefractive Glass DOI: http://dx.doi.org/10.5772/intechopen.85289*

[17] Dyamant I, Abyzov AS, Fokin VM, Zanotto ED, Lumeau J, Glebova LN, et al. Crystal nucleation and growth kinetics of NaF in photo-thermorefractive glass. Journal of Non-Crystalline Solids. 2013;**378**:115-120

[18] Efimov AM, Ignatiev AI, Nikonorov NV, Postnikov ES. Quantitative UV-VIS spectroscopic studies of photo-thermo-refractive glasses. I. Intrinsic, bromine-related, and impurity-related UV absorption in photo-thermo-refractive glass matrices. Journal of Non-Crystalline Solids. 2011;**357**(19-20):3500-3512

[19] Dubrovin VD, Ignatiev AI, Nikonorov NV. Chloride photo-thermorefractive glasses. Optical Materials Express. 2016;**6**(5):1701. Available from: https://www.osapublishing.org/abstract. cfm?URI=ome-6-5-1701

[20] Sgibnev YM, Nikonorov NV, Vasilev VN, Ignatiev AI. Optical gradient waveguides in photo-thermo-refractive glass formed by ion exchange method. Journal of Lightwave Technology. 2015;**33**(17):3730-3735

[21] Nikonorov N, Ivanov S, Dubrovin V, Ignatiev A. New photo-thermorefractive glasses for holographic optical elements: Properties and applications. In: Naydenova I, editor. Holographic Materials and Optical Systems. Dublin: InTech; 2017. pp. 435-461

[22] Lumeau J, Glebova L, Golubkov V, Zanotto ED, Glebov LB. Origin of crystallization-induced refractive index changes in photo-thermo-refractive glass. Optical Materials. 2009;**32**(1):139-146

[23] Lumeau J, Glebov LB. Effect of the refractive index change kinetics of photosensitive materials on the diffraction efficiency of reflecting Bragg gratings. Applied Optics. 2013;**52**(17):3993-3997

[24] Efimov AM, Ignatiev AI, Nikonorov NV, Postnikov ES. Quantitative UV-VIS

spectroscopic studies of photo-thermorefractive glasses. II. Manifestations of Ce3+ and Ce(IV) valence states in the UV absorption spectrum of cerium-doped photo-thermo-refractive matrix glasses. Journal of Non-Crystalline Solids. 2013;**361**(1):26-37

[25] Varotsos P. Determination of the dielectric constant of alkali halide mixed crystals. Physica Status Solidi. 1980;**100**(2):K133-K138

[26] Bruggeman VDAG. Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen. Annalen der Physik. 1935;**416**(7):636-664

[27] Kogelnik H. Coupled wave theory for thick hologram gratings. Bell System Technical Journal. 1969;**48**(9):2909-2947

[28] Carretero L, Madrigal RF, Fimia A, Blaya S, Beléndez A. Study of angular responses of mixed amplitude-phase holographic gratings: Shifted Borrmann effect. Optics Letters. 2001;**26**(11): 786-788. Available from: http://www. ncbi.nlm.nih.gov/pubmed/18040450

[29] Podlipensky AV, Grebenev V, Seifert G, Graener H. Ionization and photomodification of Ag nanoparticles in soda-lime glass by 150 fs laser irradiation: A luminescence study. Journal of Luminescence. 2004;**109**:135-142

[30] Bigot J, Halte V, Merle J, Daunois A. Electron dynamics in metallic nanoparticles. Chemical Physics. 2000;**251**:181-203

[31] Hashimoto S, Werner D, Uwada T. Studies on the interaction of pulsed lasers with plasmonic gold nanoparticles toward light manipulation, heat management, and nanofabrication. Journal of Photochemistry and Photobiology C Photochemistry Reviews. 2012;**13**(1):28-54

**18**

In Russian

*Holographic Materials and Applications*

[1] Stookey SD. Photosensetive glass. Industrial & Engineering Chemistry Research. 1949;**41**(4):856-861

glass. In: IF AN Latv. SSR, editor. Trudy VII Vsesoyuznoi Konferentsii po Radiatsionnoi Fizike i Khimii Neorganicheskikh Materialov (Proceedings of VII All-Union Conference on Radiation Physics and Chemistry of Inorganic Materials); Riga.

[11] Efimov OM, Glebov LB, Glebova LN, Richardson KC, Smirnov VI. High efficiency Bragg gratings in photothermo-refractive glass. Applied Optics.

[12] Glebov LB, Glebova LN, Richardson KA, Smirnov VI. Photo-induced processes in photo-thermo-refractive glasses. In: Soc AC, editor. Proceedings of XV Congress

[13] Ivanov SA, Ignat'ev AI, Nikonorov NV, Aseev VA. Holographic characteristics of a modified photothermorefractive glass. Journal of Optical Technology.

Tunimanova IV, Chukharev AV. Influence of glass composition on the refractive index change upon photothermo induced

Chemistry. 2001;**27**(3):241-249. Available from: http://link.springer.com/article/10.

[15] Glebova L, Lumeau J, Klimov M, Zanotto ED, Glebov LB. Role of bromine on the thermal and optical properties of photo-thermo-refractive glass. Journal of Non-Crystalline Solids. 2008;**354** (2-9):456-461. Available from: http:// linkinghub.elsevier.com/retrieve/pii/

[14] Nikonorov NV, Panysheva EI,

crystallization. Glass Physics and

1989. p. 527. In Russian

1999;**38**(2):619-627

2014;**81**(6):356-360

1023/A:1011392301107

S0022309307010903

[16] Glebov LB, Nikonorov NV,

Panysheva EI, Petrovskii GT, Savvin VV, Tunimanova IV, et al. New ways to use photosensitive glasses for recording volume phase holograms. Optics and Spectroscopy. 1992;**73**(2):237-241

on Glass. San Francisco; 1998

[2] Stookey SD, Beall GH, Pierson JE. Full-color photosensitive glass. Journal of Applied Physics. 1978;**49**(10):5114

Patent 4,057,408; 1977

**References**

Patent 4,017,318; 1977

[3] Pierson JE, Stookey SD. United States

[4] Pierson EJ, Stookey SD. United States

[6] Dotsenko AV, Efimov AM, Zakharov VK,

polychromatic. Fizika i Khimiya Stekla.

Panysheva EI, Petrovskii GT, Savvin VV,

possibilities of photosensitive glasses for the recording of volume phase diagrams. Optika i Spektroskopiya. 1992;**73**(2):404-412. In Russian

[8] Kuchinskii SA, Nikonorov NV, Panysheva EI, Savvin VV, Tunimanova I. Properties of volume phase holograms on polychromatic glasses. Optika i Spektroskopiya. 1991;**70**(6):1286-1300.

[9] Nikonorov NV, Panysheva EI.

1990. pp. 2-48. In Russian

EI, Tunimanova IV, Savvin VV,

Polychromatic glasses-a new medium for optical data recording. In: Opticheskoe Izobrazhenie i Registriruyushchie Sredy (All-Union Conference "Optical Image and Recording Media"). Leningrad: GOI;

[10] Glebov LB, Nikonorov NV, Panysheva

Tsekhomskii VA. Photothermorefractive

[5] Panysheva EI, Tunimanova IV, Tsekhomskiĭ VA. A study of coloring in polychromatic glasses. Fizika i Khimiya Stekla. 1990;**16**(2):239-244. In Russian

 Panysheva EI, Tunimanova IV. On the absorption spectra of

1985;**11**(5):592-595. In Russian

[7] Glebov LB, Nikonorov NV,

Tunimanova IV, et al. New

[32] Klyukin D, Silvennoinen M, Krykova V, Svirko Y, Sidorov A, Nikonorov N. Fluorescent clusters in chloride photo-thermo-refractive glass by femtosecond laser bleaching of Ag nanoparticles. Optics Express. 2017;**25**(11):12944

[33] Ivanov SA, Nikonorov NV, Dubrovin VD, Krykova VA. Analysis of the hologram recording on the novel chloride photo-thermo-refractive glass. In: Proceedings of SPIE. The International Society for Optical Engineering; 2017

[34] Collier RJ, Burckhardt CB, Lin LH. Diffraction from volume holograms. In: Optical Holography. Amsterdam, Netherlands: Elsevier; 1971. pp. 228-264

[35] Fally M, Ellabban M, Drevenšek-Olenik I. Out-of-phase mixed holographic gratings: A qualitative analysis. Optics Express. 2008;**16**(9):6528

**21**

**Chapter 2**

**Abstract**

**1. Introduction**

Holograms

*William Alschuler*

Emulsions, Photochemistry, and

This article reviews the range of emulsions, photochemistry, and processing techniques that have been proposed and put into practice for the successful making of display holograms. It covers various types of media including gelatin-based emulsions and photopolymers (it focuses on the former) and considers external factors that affect the final results. This is a compact review of the history of the field but focuses on the range of easily available commercial emulsions, as well as certain accounts of how to make holographic emulsions from scratch. It considers various combinations of developer, bleach, and redeveloper, which have been used to achieve the best of various trade-offs for such factors as resolution, contrast, diffraction efficiency, clarity, color quality, blackest blacks, and resistance to printout.

It describes a recent advance in hypersensitizing holographic emulsions.

Display holography at its birth was not worth a second glance. Dennis Gabor's first images to demonstrate his new principle of image recording with scattered light were of flat objects, two-dimensional text on transparent film [1]. His idea had grown out of a desire to find a way to make an X-ray microscope, in an era when focusing X-ray optics did not exist. He conceived a way to use monoenergetic (monochromatic) X-ray scattering off objects to interfere with directly delivered X-rays of the same wavelength to create an interference pattern that coded information of the object's transparency, texture, and disposition in space, which could be recorded, and, when viewed in similar illumination, reveal a reconstructed image of the object. At that time he not only lacked focusing optics but also a medium with sufficient resolution to record the tiny-scale interference pattern details. As a demonstration he chose to use a mercury vapor discharge tube as a source, filtered to pass just one spectrum line, and photographic film to record the hologram. The limited spectral purity of the source only permitted an exposure setup with the source, filter, transparency object, and film in a straight line. This arrangement provided an interference pattern of maximum scale, one possible to record on film of ordinary resolution. The view of the hologram image seen through the

**Keywords:** holograms, holographic emulsions, hologram processing,

hologram exposure factors, hologram hypersensitizing

Processing Factors for Display

#### **Chapter 2**

*Holographic Materials and Applications*

[32] Klyukin D, Silvennoinen M, Krykova V, Svirko Y, Sidorov A, Nikonorov N. Fluorescent clusters in chloride photo-thermo-refractive glass by femtosecond laser bleaching of Ag nanoparticles. Optics Express.

[33] Ivanov SA, Nikonorov NV, Dubrovin VD, Krykova VA. Analysis of the hologram recording on the novel chloride photo-thermo-refractive glass. In: Proceedings of SPIE. The International Society for Optical

[34] Collier RJ, Burckhardt CB, Lin LH. Diffraction from volume holograms. In: Optical Holography. Amsterdam, Netherlands: Elsevier; 1971. pp. 228-264

2017;**25**(11):12944

Engineering; 2017

[35] Fally M, Ellabban M,

2008;**16**(9):6528

Drevenšek-Olenik I. Out-of-phase mixed holographic gratings: A qualitative analysis. Optics Express.

**20**

## Emulsions, Photochemistry, and Processing Factors for Display Holograms

*William Alschuler*

#### **Abstract**

This article reviews the range of emulsions, photochemistry, and processing techniques that have been proposed and put into practice for the successful making of display holograms. It covers various types of media including gelatin-based emulsions and photopolymers (it focuses on the former) and considers external factors that affect the final results. This is a compact review of the history of the field but focuses on the range of easily available commercial emulsions, as well as certain accounts of how to make holographic emulsions from scratch. It considers various combinations of developer, bleach, and redeveloper, which have been used to achieve the best of various trade-offs for such factors as resolution, contrast, diffraction efficiency, clarity, color quality, blackest blacks, and resistance to printout. It describes a recent advance in hypersensitizing holographic emulsions.

**Keywords:** holograms, holographic emulsions, hologram processing, hologram exposure factors, hologram hypersensitizing

#### **1. Introduction**

Display holography at its birth was not worth a second glance. Dennis Gabor's first images to demonstrate his new principle of image recording with scattered light were of flat objects, two-dimensional text on transparent film [1]. His idea had grown out of a desire to find a way to make an X-ray microscope, in an era when focusing X-ray optics did not exist. He conceived a way to use monoenergetic (monochromatic) X-ray scattering off objects to interfere with directly delivered X-rays of the same wavelength to create an interference pattern that coded information of the object's transparency, texture, and disposition in space, which could be recorded, and, when viewed in similar illumination, reveal a reconstructed image of the object. At that time he not only lacked focusing optics but also a medium with sufficient resolution to record the tiny-scale interference pattern details. As a demonstration he chose to use a mercury vapor discharge tube as a source, filtered to pass just one spectrum line, and photographic film to record the hologram. The limited spectral purity of the source only permitted an exposure setup with the source, filter, transparency object, and film in a straight line. This arrangement provided an interference pattern of maximum scale, one possible to record on film of ordinary resolution. The view of the hologram image seen through the

hologram was a twin image centered on the light source. This is not a favorable view for display holography. However, it demonstrated that the principle was correct: an interference pattern can record details of the object, and its recording could reconstruct an image of the object when the viewing light is in the location that it occupied when the recording was made.

The game changed with the invention of lasers in 1961. Almost immediately, Leith and Upatnieks, who had been working on imaging from side-looking radar and had adopted a version of Gabor's invention to do so, realized that the laser's orders of magnitude better purity allowed an off-axis arrangement for the exposing setup and thus a view of the object without the light source mixed in [2]. They published an article with photos of the images they made with this technique, clearly demonstrating potential for realistic 3-D images that can be viewed without any optical aid [3].

Now, the production of suitable emulsions added a new requirement: it must resolve the interference pattern that, due to the change in beam geometry, became much finer, in certain respects, down to half (or less) of the wavelength of the laser light chosen. This requirement amounts to resolving at least 3000 lines per mm (lpmm) for red, 5000 lpmm for green, and at least 8,000 lpmm for blue. The grain size in the last case would need to be around 10–20 nm. These numbers contrast with resolutions of 75–150 lpmm for conventional films.

This can be done and in fact was demonstrated to be achievable back in the 1890s, by Lippmann [4, 5]. Lippmann created such an emulsion in pursuit of recording the interference pattern of ordinary light interfering with itself upon reflection at the surface of the emulsion, in contact with a mercury mirror. In principle it was the same as a reflection hologram, with full color but with no 3-D. It won him the Nobel Prize in Physics in 1908.

Ever since the work of Leith and Upatnieks, the search for emulsion materials, photochemistry, and processing has had the goals of making possible holograms with great brightness, color saturation, sharpness, contrast, clarity, minimum noise in the form of scattering, resistance to printout from further exposure to light, robustness of the emulsion in and after processing, and process safety. In these goals hides a fundamental conflict well known in ordinary photography: the inverse relationship between resolving power/grain size and sensitivity—the finer the grain, the less sensitive the emulsion. Many holographic emulsions have speeds that would lie in the range of ISO 1 or less.

It is striking that this challenge has existed in some form for the whole history of photography, the last 180 years, and that the generally best emulsion for meeting the challenge is some form of silver halide in gelatin, which has existed almost as long. Newer emulsions have come into existence that have proven useful for certain types and uses of holograms, but overall silver halides in gelatin still dominate display holography. This survey looks at some alternatives but is interested in versions of the latter, including some recent progress.

#### **2. The range of emulsion types**

In the history of holographic emulsions, silver halides in gelatin came first, as the dominating photo emulsion of the time. Then, dichromated gelatin, which was used in two old processes, carbon and gum bichromate, came into play in holography for uses that demanded the brightest possible images or greatest efficiency in focusing or diffracting light in holographic optical elements (HOEs). They are also used as master holograms for the production of mass market-reproduced holograms. The third class of materials, which includes photoresists and the possibly wide range of

**23**

devices.

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

They are more physically robust than those in dichromated gelatin.

photopolymers, came into use for HOEs and also for masters for the mass market.

Unlike all the materials listed above, which have positive indices of refraction, there is a class of materials new to visible light photonics that has a negative index of refraction. This class of materials, now called metamaterials, has moderately distant roots in some research, going back as far as the nineteenth century, with seriously renewed work in the 1950s to control radiation at radio- and microwave wavelengths [6]. The materials are artificially constructed patterned dielectrics and mixed metals and dielectrics, with structural scale at or smaller than the wavelength in question. They have been constructed to investigate various possibilities that can be envisioned that are not open to ordinary, positive-index materials. The optical properties depend on the shape and spacing of the elements of the pattern, more than the intrinsic optical properties of the materials they are constructed of. Best known of the optical element ideas is the "invisibility cloak" (think of Harry Potter's invisibility cloak), a material that can bend light so that it hides any light from what it encloses and routes the light from the background around to the front. This has been demonstrated for single wavelengths, but it is likely to be a long time before such a material can be made that creates this functionality across the whole visible spectrum. Even with this single-frequency embodiment, such a cloak could have been used in evading detection at specified wavelengths from radar-like

Nonetheless, work proceeds on creating metamaterial holograms. These have so far been the versions of HOEs rather than images of ordinary subjects, because the patterns are computed before being deposited on a surface, and such things as lenses and mirrors and diffraction gratings are both computationally and depositionally simple. Groups at Duke University have designed and made holograms that store two images in the same volume, independently viewable by illuminating the hologram with light polarized at the orientation the pattern was designed for [7]. A group at the University of Pennsylvania reports constructing a flexible metamaterial made of polydimethylsiloxane (PDMS) with embedded gold rods set in precise patterns. Three images were embedded by designing three patterns, which become

There are three types of information storage in holograms made in original exposures in photosensitive media. If the information is recorded as opacity and transparency patterns, this is termed an amplitude recording. If the information is in the form of patterns of varying index of refraction, this is termed a phase recording. Third, the information may be stored as microscopic surface relief patterns. Diffraction efficiency, the ratio of the energy provided in the image to the energy incident on the hologram at its viewing, is a primary determinant of the image brightness. It has been shown theoretically that phase-only holograms as well as phase combined with surface relief can achieve diffraction efficiencies close to 100%. Amplitude holograms have been shown to be limited in diffraction efficiency to about 50%. Practice over the years, in the various media types and recording

Copying holograms for small editions or mass production faces the same limits.

This category of materials is basically plastics, which by various mechanisms start as monomers and then respond to light exposure by changing to polymers.

**2.2 Photopolymer, photoresistive, and photothermoplastic emulsions**

separately viewable as the sheet is stretched and illuminated [8].

**2.1 Information recording types and diffraction efficiencies**

schemes, shows these theoretical estimates to be correct [9].

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

#### *Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

*Holographic Materials and Applications*

occupied when the recording was made.

with resolutions of 75–150 lpmm for conventional films.

won him the Nobel Prize in Physics in 1908.

would lie in the range of ISO 1 or less.

of the latter, including some recent progress.

**2. The range of emulsion types**

any optical aid [3].

hologram was a twin image centered on the light source. This is not a favorable view for display holography. However, it demonstrated that the principle was correct: an interference pattern can record details of the object, and its recording could reconstruct an image of the object when the viewing light is in the location that it

The game changed with the invention of lasers in 1961. Almost immediately, Leith and Upatnieks, who had been working on imaging from side-looking radar and had adopted a version of Gabor's invention to do so, realized that the laser's orders of magnitude better purity allowed an off-axis arrangement for the exposing setup and thus a view of the object without the light source mixed in [2]. They published an article with photos of the images they made with this technique, clearly demonstrating potential for realistic 3-D images that can be viewed without

Now, the production of suitable emulsions added a new requirement: it must resolve the interference pattern that, due to the change in beam geometry, became much finer, in certain respects, down to half (or less) of the wavelength of the laser light chosen. This requirement amounts to resolving at least 3000 lines per mm (lpmm) for red, 5000 lpmm for green, and at least 8,000 lpmm for blue. The grain size in the last case would need to be around 10–20 nm. These numbers contrast

This can be done and in fact was demonstrated to be achievable back in the 1890s, by Lippmann [4, 5]. Lippmann created such an emulsion in pursuit of recording the interference pattern of ordinary light interfering with itself upon reflection at the surface of the emulsion, in contact with a mercury mirror. In principle it was the same as a reflection hologram, with full color but with no 3-D. It

Ever since the work of Leith and Upatnieks, the search for emulsion materials, photochemistry, and processing has had the goals of making possible holograms with great brightness, color saturation, sharpness, contrast, clarity, minimum noise in the form of scattering, resistance to printout from further exposure to light, robustness of the emulsion in and after processing, and process safety. In these goals hides a fundamental conflict well known in ordinary photography: the inverse relationship between resolving power/grain size and sensitivity—the finer the grain, the less sensitive the emulsion. Many holographic emulsions have speeds that

It is striking that this challenge has existed in some form for the whole history of photography, the last 180 years, and that the generally best emulsion for meeting the challenge is some form of silver halide in gelatin, which has existed almost as long. Newer emulsions have come into existence that have proven useful for certain types and uses of holograms, but overall silver halides in gelatin still dominate display holography. This survey looks at some alternatives but is interested in versions

In the history of holographic emulsions, silver halides in gelatin came first, as the dominating photo emulsion of the time. Then, dichromated gelatin, which was used in two old processes, carbon and gum bichromate, came into play in holography for uses that demanded the brightest possible images or greatest efficiency in focusing or diffracting light in holographic optical elements (HOEs). They are also used as master holograms for the production of mass market-reproduced holograms. The third class of materials, which includes photoresists and the possibly wide range of

**22**

photopolymers, came into use for HOEs and also for masters for the mass market. They are more physically robust than those in dichromated gelatin.

Unlike all the materials listed above, which have positive indices of refraction, there is a class of materials new to visible light photonics that has a negative index of refraction. This class of materials, now called metamaterials, has moderately distant roots in some research, going back as far as the nineteenth century, with seriously renewed work in the 1950s to control radiation at radio- and microwave wavelengths [6]. The materials are artificially constructed patterned dielectrics and mixed metals and dielectrics, with structural scale at or smaller than the wavelength in question. They have been constructed to investigate various possibilities that can be envisioned that are not open to ordinary, positive-index materials. The optical properties depend on the shape and spacing of the elements of the pattern, more than the intrinsic optical properties of the materials they are constructed of. Best known of the optical element ideas is the "invisibility cloak" (think of Harry Potter's invisibility cloak), a material that can bend light so that it hides any light from what it encloses and routes the light from the background around to the front. This has been demonstrated for single wavelengths, but it is likely to be a long time before such a material can be made that creates this functionality across the whole visible spectrum. Even with this single-frequency embodiment, such a cloak could have been used in evading detection at specified wavelengths from radar-like devices.

Nonetheless, work proceeds on creating metamaterial holograms. These have so far been the versions of HOEs rather than images of ordinary subjects, because the patterns are computed before being deposited on a surface, and such things as lenses and mirrors and diffraction gratings are both computationally and depositionally simple. Groups at Duke University have designed and made holograms that store two images in the same volume, independently viewable by illuminating the hologram with light polarized at the orientation the pattern was designed for [7]. A group at the University of Pennsylvania reports constructing a flexible metamaterial made of polydimethylsiloxane (PDMS) with embedded gold rods set in precise patterns. Three images were embedded by designing three patterns, which become separately viewable as the sheet is stretched and illuminated [8].

#### **2.1 Information recording types and diffraction efficiencies**

There are three types of information storage in holograms made in original exposures in photosensitive media. If the information is recorded as opacity and transparency patterns, this is termed an amplitude recording. If the information is in the form of patterns of varying index of refraction, this is termed a phase recording. Third, the information may be stored as microscopic surface relief patterns.

Diffraction efficiency, the ratio of the energy provided in the image to the energy incident on the hologram at its viewing, is a primary determinant of the image brightness. It has been shown theoretically that phase-only holograms as well as phase combined with surface relief can achieve diffraction efficiencies close to 100%. Amplitude holograms have been shown to be limited in diffraction efficiency to about 50%. Practice over the years, in the various media types and recording schemes, shows these theoretical estimates to be correct [9].

Copying holograms for small editions or mass production faces the same limits.

#### **2.2 Photopolymer, photoresistive, and photothermoplastic emulsions**

This category of materials is basically plastics, which by various mechanisms start as monomers and then respond to light exposure by changing to polymers.

#### *2.2.1 Photoresists*

These substances, rather varied in composition, upon exposure to blue or violet light become soluble or insoluble when soaked in an organic solvent. The coating rests on a substrate, and usually the coating thickness is fairly small, one to two microns. The exposures required are up to 100 mJ/cm2 of emulsion. These materials can be used to generate master holograms for embossed replica holograms.

This type of process recalls one of the two very earliest processes of photography: Niepce's 1827 process that employed Bitumen of Judea, coated onto a copper plate, as the photosensitive emulsion. Exposure to light caused it to polymerize and harden. Where it had not been exposed, when placed in a bath of oil of lavender, the Bitumen dissolved. The result was a relief plate that could be inked and run through a printing press.

#### *2.2.2 Photothermoplastics*

These materials typically have a multilayer structure that starts with a glass substrate; then a transparent conductive thin film, usually indium oxide; then copper contacts at the perimeter of the conductive layer; then a photoconductive layer, usually 2 microns thick; and finally on top is the thermoplastic layer, generally a fraction of micron thick, say three tenths of a micron. The surface of the thermoplastic is given a positive electric charge at its exposed surface; it is exposed to light, either blue or violet. This causes charges to migrate across the surface to form the pattern of exposure. A current is passed through the conductor to heat the thermoplastic, and it responds by forming a surface relief reproduction of the hologram interference pattern. The hologram is then ready for viewing. By heating the thermoplastic again, the charge redistributes itself to become uniform, the surface relief disappears, the image disappears, and the material is ready for use again [10].

#### *2.2.3 Photopolymers*

Photopolymers generally respond to exposure to light by polymerizing, and this generally results in changes to their density and thus their index of refraction that reproduces the interference pattern, mostly inside the emulsion. Photopolymers find a range of uses outside of holography, including some cyanoacrylate glues, bonding cement for glass, and UV-curing dental cements.

Holograms on photopolymers have a number of applications, most of which are not for display holography. These applications include security enhancement on credit cards and banknotes (so far the largest part of the market), occasionally for "flash" on packaging, makeup, trading cards, and large-volume replica images. They are also used for HOEs, such as for solar illumination control, concentrators for solar heat and photovoltaic collectors, optics for head-up displays and enhanced reality glasses, selective reflectors for laser protective glasses, optical computing, high-density optical data storage, and rapid pattern recognition.

For these as for all volume holograms, the larger the thickness, the narrower the wavelength bandwidths and also the narrower the viewing angles. In these properties volume holograms act as Bragg gratings. Narrow viewing angles are useful in many situations where holograms are used in HOEs, but usually not desirable for display holograms, where a wide audience is often a goal.

The active ingredient in the film is a photosensitive monomer that polymerizes to form the recording. The other components of a typical photopolymer film include binders (polymers that act to structure the film); photoinitiators, often

**25**

went into bankruptcy.

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

various sensitizing dyes, which begin the monomer polymerization; and plasticiz-

The gross physical structure is usually in three parts: a cover sheet that can easily be peeled off, the active photopolymer layer, and a base sheet that normally remains attached after finishing the exposure. The base sheet optical properties must be paid attention to, specifically their transparency and amount of birefringence.

In use the cover sheet is first peeled off, and then the photopolymer side, which is sticky, is hand applied to a carefully cleaned glass sheet with the help of a roller. Care is taken to avoid and remove trapped air bubbles. Also, care is taken to be dust

The photopolymerization process is usually described on a molecular scale as starting with the absorption of light by the sensitizing dyes and active monomers. Then, the dyes linked to the monomers or other initiators generate free radicals. The free radicals capture and combine the monomers into polymers. As the process proceeds, free monomers diffuse out of their original positions into the regions where the polymers are forming. Their arrival in that area and evacuation from their original sites create density variations that in turn give rise to the variations in the index of refraction that ultimately constitute the hologram [11]. For some compositions of film, there should be as short an interval of time between exposure and heat or UV treatment as possible to avoid reaction reversal and image fading. Related to this last issue is the fact that one of a volume hologram's desirable properties is the ability to overlay multiple images in the same volume and then to view them separately. This is the basis for high-capacity holographic memory. In order to achieve the desired goal of as many stored images as possible with uniform optical quality, it is necessary to take into account the fact that each successive exposure will tend to overwrite the preceding ones, both modifying the earlier records and eventually using up all monomers and saturating the film. Generally, later records will be weaker than earlier ones. Self-absorption by earlier records will also affect laser beam intensity of later ones, also affecting the achieved diffraction efficiency. A strategy to deal with this can be devised in the form of an exposure schedule. Assuming a constant intensity per exposure, predetermined schedule ramps up the exposure in a slightly nonlinear fashion, so that the last exposure of

One of the earliest and most important commercial families of photopolymers for holography was developed by E. I. duPont. It started with HRF-700, and now there are later versions, which are stable; have an index of refraction modulation of 0.06, which is quite high; and can be made in thicknesses up to 100 microns [11]. Dupont distributed film samples to the wider holographic community, including those who made display holograms, for a number of years, but eventually changed its policy and restricted distribution to a select "approved" list, which is now basi-

There are safety issues that can occur with the acrylate monomers that are often used in these films and specifically in Dupont's films. After exposure the films are UV and heat cured. Though most monomer content will typically have been converted to polymers in the exposure and UV after-exposure, the latter step can generate some acrylate monomer vapors. Therefore, curing ovens should be ventilated to the outdoors. If the vapor condenses indoors on surfaces, including the skin, it will be an oily liquid and should be removed as quickly as possible. Handwashing with soap and water is recommended at every shift. This and other directives can be

Polaroid developed and for a time marketed a high-quality photopolymer that was available in thicknesses up to 1 mm [15]. It was discontinued when Polaroid

cally limited to companies using the films for mass production [13].

found in Dupont's literature on handling their films [14].

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

free, at least until the image is finished [11].

350 is four times the exposure of the first [12].

ers, which improve film flexibility.

#### *Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

*Holographic Materials and Applications*

microns. The exposures required are up to 100 mJ/cm2

These substances, rather varied in composition, upon exposure to blue or violet light become soluble or insoluble when soaked in an organic solvent. The coating rests on a substrate, and usually the coating thickness is fairly small, one to two

This type of process recalls one of the two very earliest processes of photography: Niepce's 1827 process that employed Bitumen of Judea, coated onto a copper plate, as the photosensitive emulsion. Exposure to light caused it to polymerize and harden. Where it had not been exposed, when placed in a bath of oil of lavender, the Bitumen dissolved. The result was a relief plate that could be inked and run through

These materials typically have a multilayer structure that starts with a glass substrate; then a transparent conductive thin film, usually indium oxide; then copper contacts at the perimeter of the conductive layer; then a photoconductive layer, usually 2 microns thick; and finally on top is the thermoplastic layer, generally a fraction of micron thick, say three tenths of a micron. The surface of the thermoplastic is given a positive electric charge at its exposed surface; it is exposed to light, either blue or violet. This causes charges to migrate across the surface to form the pattern of exposure. A current is passed through the conductor to heat the thermoplastic, and it responds by forming a surface relief reproduction of the hologram interference pattern. The hologram is then ready for viewing. By heating the thermoplastic again, the charge redistributes itself to become uniform, the surface relief disappears, the image disappears, and the material is ready for use

Photopolymers generally respond to exposure to light by polymerizing, and this generally results in changes to their density and thus their index of refraction that reproduces the interference pattern, mostly inside the emulsion. Photopolymers find a range of uses outside of holography, including some cyanoacrylate glues,

Holograms on photopolymers have a number of applications, most of which are not for display holography. These applications include security enhancement on credit cards and banknotes (so far the largest part of the market), occasionally for "flash" on packaging, makeup, trading cards, and large-volume replica images. They are also used for HOEs, such as for solar illumination control, concentrators for solar heat and photovoltaic collectors, optics for head-up displays and enhanced reality glasses, selective reflectors for laser protective glasses, optical computing,

For these as for all volume holograms, the larger the thickness, the narrower the wavelength bandwidths and also the narrower the viewing angles. In these properties volume holograms act as Bragg gratings. Narrow viewing angles are useful in many situations where holograms are used in HOEs, but usually not desirable for

The active ingredient in the film is a photosensitive monomer that polymerizes to form the recording. The other components of a typical photopolymer film include binders (polymers that act to structure the film); photoinitiators, often

bonding cement for glass, and UV-curing dental cements.

high-density optical data storage, and rapid pattern recognition.

display holograms, where a wide audience is often a goal.

can be used to generate master holograms for embossed replica holograms.

of emulsion. These materials

*2.2.1 Photoresists*

a printing press.

again [10].

*2.2.3 Photopolymers*

*2.2.2 Photothermoplastics*

**24**

various sensitizing dyes, which begin the monomer polymerization; and plasticizers, which improve film flexibility.

The gross physical structure is usually in three parts: a cover sheet that can easily be peeled off, the active photopolymer layer, and a base sheet that normally remains attached after finishing the exposure. The base sheet optical properties must be paid attention to, specifically their transparency and amount of birefringence.

In use the cover sheet is first peeled off, and then the photopolymer side, which is sticky, is hand applied to a carefully cleaned glass sheet with the help of a roller. Care is taken to avoid and remove trapped air bubbles. Also, care is taken to be dust free, at least until the image is finished [11].

The photopolymerization process is usually described on a molecular scale as starting with the absorption of light by the sensitizing dyes and active monomers. Then, the dyes linked to the monomers or other initiators generate free radicals. The free radicals capture and combine the monomers into polymers. As the process proceeds, free monomers diffuse out of their original positions into the regions where the polymers are forming. Their arrival in that area and evacuation from their original sites create density variations that in turn give rise to the variations in the index of refraction that ultimately constitute the hologram [11]. For some compositions of film, there should be as short an interval of time between exposure and heat or UV treatment as possible to avoid reaction reversal and image fading.

Related to this last issue is the fact that one of a volume hologram's desirable properties is the ability to overlay multiple images in the same volume and then to view them separately. This is the basis for high-capacity holographic memory. In order to achieve the desired goal of as many stored images as possible with uniform optical quality, it is necessary to take into account the fact that each successive exposure will tend to overwrite the preceding ones, both modifying the earlier records and eventually using up all monomers and saturating the film. Generally, later records will be weaker than earlier ones. Self-absorption by earlier records will also affect laser beam intensity of later ones, also affecting the achieved diffraction efficiency. A strategy to deal with this can be devised in the form of an exposure schedule. Assuming a constant intensity per exposure, predetermined schedule ramps up the exposure in a slightly nonlinear fashion, so that the last exposure of 350 is four times the exposure of the first [12].

One of the earliest and most important commercial families of photopolymers for holography was developed by E. I. duPont. It started with HRF-700, and now there are later versions, which are stable; have an index of refraction modulation of 0.06, which is quite high; and can be made in thicknesses up to 100 microns [11].

Dupont distributed film samples to the wider holographic community, including those who made display holograms, for a number of years, but eventually changed its policy and restricted distribution to a select "approved" list, which is now basically limited to companies using the films for mass production [13].

There are safety issues that can occur with the acrylate monomers that are often used in these films and specifically in Dupont's films. After exposure the films are UV and heat cured. Though most monomer content will typically have been converted to polymers in the exposure and UV after-exposure, the latter step can generate some acrylate monomer vapors. Therefore, curing ovens should be ventilated to the outdoors. If the vapor condenses indoors on surfaces, including the skin, it will be an oily liquid and should be removed as quickly as possible. Handwashing with soap and water is recommended at every shift. This and other directives can be found in Dupont's literature on handling their films [14].

Polaroid developed and for a time marketed a high-quality photopolymer that was available in thicknesses up to 1 mm [15]. It was discontinued when Polaroid went into bankruptcy.

There are two more recent developments in photopolymers that are worth noting.

The first is a relatively new film family by Bayer, trademarked Bayfol HX. It was supplied at least initially at 16 microns thickness, requires no postexposure heat treatment, and has an index modulation of 0.030 at the exposure wavelengths of 633, 532, and 473 nm. Typical exposure energies are 18, 25, and 30 mJ, respectively [16, 17]. It has been supplied as a four-layer film. The first is a removable cover film, the second the photopolymer, the third the substrate, and the last another removable cover film. The substrates have included low-birefringence polycarbonate and polyethylene terephthalate.

The photopolymer is a two-chemistry composite. The first is the matrix precursors, which get cross-linked during manufacturing and thereby form a matrix that provides stability. The second is the photosensitive part of the film and includes the absorbers (dyes), the initiators, and the monomers. The unexposed film can be stored and shipped (in the dark) if kept at reasonably temperature-stable conditions. During exposure the image forms. After sufficient exposure to the lasers, the film is photo-cured to bleach out the absorbers and improve transparency. No heat treatment is needed. Bayer claims that with the two "independent" photochemistries, each can be separately optimized to allow for various quality requirements.

The second development is that recently Liti Holographics entered the market with photopolymer-based kits for home and school holography and also as a supplier of large holographic images for advertising. The film is marketed for home and school use as C-RT20 in 2 × 3 inch and 4 × 5 inch sizes in packets of 6–20. The commercial services also include offers to professional photographers for add-on services for portrait photography. They promise saturated color, large view zones (120°), 7 seconds of animation, and forward projecting (i.e., real) images [18–20]. This film develops as it is exposed, so that the image slowly comes into view (in early photographic terminology, it "prints out"). It requires no further processing. The fact that this film is self-developing and requires no chemical processing is a definite plus in terms of safety, especially for home and educational uses.

The active film is specified as being 16 microns thick with a 50 micron ®cover sheet and a 1.8 mm substrate. It claims diffraction efficiencies of 99% and to be panchromatic, requiring exposures of 20, 30, and 50 mJ/cm2 at 635, 532, and 450 nm, respectively. It is optional to post expose the film to white light that will bleach out remaining monomers and improve clarity [21].

#### *2.2.3.1 Photopolymer holographic applications to augmented reality displays*

In recent reports various applications of photopolymer holograms to augmented reality displays are described. In one design a hologram is used to provide a selectively reflecting optical element for use in eyeglass-type head-up 2-D displays [22]. The hologram is constructed as a selectively reflecting hologram, designed to create a view directly in front of the eyes, using various locations and types of illumination geometries. These include straightforward down looking informational screens, such as LCDs, and also embedding the holoscreen in a larger edge-illuminated slab, so that the projected image arrives by repeated total internal reflection at the holoscreen, which then reflects it out of the slab to the eye.

In the second, a 3-D see-through screen is achieved by a conversion of Lippmann's 1908 [23] idea of an integral photograph lenslet array to a holographic version. The original idea achieved a 3-D view of a scene by photographing it through a fly's eye lens array with the recording emulsion at the back focal plane of the array. After development and reversal to a positive, it was replaced behind the array and viewed through the array. The lenslets each provided their own views

**27**

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

of the scene, which in total gave full parallax, both horizontal and vertical, for the viewer. In Lippmann's time producing such an array was not simple, and he apparently only got one custom-made example with which he confirmed his idea. The new version [24] converts a lens array to a photopolymer hologram of the reflection or Denisyuk type to take advantage both of the transparency of the medium, which allows see-through views of the world beyond, and of the angular and wavelength selectivity of the hologram, which is a "thick" Bragg-angle reflector that is very wavelength selective, when illuminated with ordinary incoherent light. (Transmission-type holograms also work, but are not wavelength selective.) Illuminated at the reference beam angle, the diffraction efficiency can easily be 30%, but at the same time, the transmission through the holographic array is up around 90%. Using R, G, and B lasers to record the array, it will render full-color

When the reference beam is replaced by a projected view of information, for example, projected from a spatial light modulator, the array forms the reflected and focused image through which the world is seen. The illumination angle can be chosen to use the phase-reversed version of the hologram. This avoids the problem of reversed depth perspective in the real image (pseudoscopy) and gives an ortho-

Various schemes for multiplexing 2-D and 3-D images are also explored, as are schemes for enlarging the image viewing zone. To implement large screens, which run into problems of high required laser powers and large optical systems to project the array, it is suggested that the array be constructed by mosaicking smaller arrays side by side on the polymer, and the reference beam can be diverging, instead of a

Among the various applications, it seems that in principle the screen could be very large and allow projection of 3-D images in a Pepper's Ghost arrangement for

A holographic recording material that apparently can be switched on and off by application of an electric current is mentioned in Ref. [25] by Bob Hess. It is described as "Holographic Polymer Dispersed Liquid Crystal (HPLDC) produced by SBG Labs (formerly DigiLens, formerly Retinal Displays)." At least one use already in production is for augmented reality helmet optics according to the DigiLens website, and there it is described as a switchable Bragg grating [26]. The grating structure is synthesized according to the optical element functionality desired.

*2.2.5 A very exotic, distinctive (and tasty) family of surface relief holographic media*

Back in 1997, after there was publicly available experience with photopolymers and dichromated gelatin that recorded surface relief images of the hologram interference patterns that allowed development of mass replication of master holograms, an entrepreneur experimented with and eventually succeeded in impressing a surface relief hologram on the surface of chocolate bars. To do this he started with a metal shim, made from a conventional relief master hologram, and used it to stamp copies in the chocolate. He had to find the right chocolate rigidity to record and retain the patterns, as is also needed, but perhaps less difficult to achieve in making conventional plastic copies [27]. Of course, the image quality would suffer if the chocolate went through a long enough experience of high enough temperature or was pressed down upon or rubbed in handling. This market was revisited more recently and expanded to include hard candies in addition to chocolate [28].

large audiences. However, there likely would be practical difficulties.

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

images, assuming the wavelengths are well chosen.

scopic version in a virtual image.

*2.2.4 An exotic potential development*

collimated parallel beam.

#### *Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

*Holographic Materials and Applications*

polyethylene terephthalate.

noting.

There are two more recent developments in photopolymers that are worth

The first is a relatively new film family by Bayer, trademarked Bayfol HX. It was supplied at least initially at 16 microns thickness, requires no postexposure heat treatment, and has an index modulation of 0.030 at the exposure wavelengths of 633, 532, and 473 nm. Typical exposure energies are 18, 25, and 30 mJ, respectively [16, 17]. It has been supplied as a four-layer film. The first is a removable cover film, the second the photopolymer, the third the substrate, and the last another removable cover film. The substrates have included low-birefringence polycarbonate and

The photopolymer is a two-chemistry composite. The first is the matrix precursors, which get cross-linked during manufacturing and thereby form a matrix that provides stability. The second is the photosensitive part of the film and includes the absorbers (dyes), the initiators, and the monomers. The unexposed film can be stored and shipped (in the dark) if kept at reasonably temperature-stable conditions. During exposure the image forms. After sufficient exposure to the lasers, the film is photo-cured to bleach out the absorbers and improve transparency. No heat treatment is needed. Bayer claims that with the two "independent" photochemistries, each can be separately optimized to allow for various quality requirements. The second development is that recently Liti Holographics entered the market with photopolymer-based kits for home and school holography and also as a supplier of large holographic images for advertising. The film is marketed for home and school use as C-RT20 in 2 × 3 inch and 4 × 5 inch sizes in packets of 6–20. The commercial services also include offers to professional photographers for add-on services for portrait photography. They promise saturated color, large view zones (120°), 7 seconds of animation, and forward projecting (i.e., real) images [18–20]. This film develops as it is exposed, so that the image slowly comes into view (in early photographic terminology, it "prints out"). It requires no further processing. The fact that this film is self-developing and requires no chemical processing is a

definite plus in terms of safety, especially for home and educational uses.

*2.2.3.1 Photopolymer holographic applications to augmented reality displays*

In the second, a 3-D see-through screen is achieved by a conversion of Lippmann's 1908 [23] idea of an integral photograph lenslet array to a holographic version. The original idea achieved a 3-D view of a scene by photographing it through a fly's eye lens array with the recording emulsion at the back focal plane of the array. After development and reversal to a positive, it was replaced behind the array and viewed through the array. The lenslets each provided their own views

panchromatic, requiring exposures of 20, 30, and 50 mJ/cm2

bleach out remaining monomers and improve clarity [21].

holoscreen, which then reflects it out of the slab to the eye.

The active film is specified as being 16 microns thick with a 50 micron ®cover sheet and a 1.8 mm substrate. It claims diffraction efficiencies of 99% and to be

In recent reports various applications of photopolymer holograms to augmented reality displays are described. In one design a hologram is used to provide a selectively reflecting optical element for use in eyeglass-type head-up 2-D displays [22]. The hologram is constructed as a selectively reflecting hologram, designed to create a view directly in front of the eyes, using various locations and types of illumination geometries. These include straightforward down looking informational screens, such as LCDs, and also embedding the holoscreen in a larger edge-illuminated slab, so that the projected image arrives by repeated total internal reflection at the

450 nm, respectively. It is optional to post expose the film to white light that will

at 635, 532, and

**26**

of the scene, which in total gave full parallax, both horizontal and vertical, for the viewer. In Lippmann's time producing such an array was not simple, and he apparently only got one custom-made example with which he confirmed his idea.

The new version [24] converts a lens array to a photopolymer hologram of the reflection or Denisyuk type to take advantage both of the transparency of the medium, which allows see-through views of the world beyond, and of the angular and wavelength selectivity of the hologram, which is a "thick" Bragg-angle reflector that is very wavelength selective, when illuminated with ordinary incoherent light. (Transmission-type holograms also work, but are not wavelength selective.) Illuminated at the reference beam angle, the diffraction efficiency can easily be 30%, but at the same time, the transmission through the holographic array is up around 90%. Using R, G, and B lasers to record the array, it will render full-color images, assuming the wavelengths are well chosen.

When the reference beam is replaced by a projected view of information, for example, projected from a spatial light modulator, the array forms the reflected and focused image through which the world is seen. The illumination angle can be chosen to use the phase-reversed version of the hologram. This avoids the problem of reversed depth perspective in the real image (pseudoscopy) and gives an orthoscopic version in a virtual image.

Various schemes for multiplexing 2-D and 3-D images are also explored, as are schemes for enlarging the image viewing zone. To implement large screens, which run into problems of high required laser powers and large optical systems to project the array, it is suggested that the array be constructed by mosaicking smaller arrays side by side on the polymer, and the reference beam can be diverging, instead of a collimated parallel beam.

Among the various applications, it seems that in principle the screen could be very large and allow projection of 3-D images in a Pepper's Ghost arrangement for large audiences. However, there likely would be practical difficulties.

#### *2.2.4 An exotic potential development*

A holographic recording material that apparently can be switched on and off by application of an electric current is mentioned in Ref. [25] by Bob Hess. It is described as "Holographic Polymer Dispersed Liquid Crystal (HPLDC) produced by SBG Labs (formerly DigiLens, formerly Retinal Displays)." At least one use already in production is for augmented reality helmet optics according to the DigiLens website, and there it is described as a switchable Bragg grating [26]. The grating structure is synthesized according to the optical element functionality desired.

#### *2.2.5 A very exotic, distinctive (and tasty) family of surface relief holographic media*

Back in 1997, after there was publicly available experience with photopolymers and dichromated gelatin that recorded surface relief images of the hologram interference patterns that allowed development of mass replication of master holograms, an entrepreneur experimented with and eventually succeeded in impressing a surface relief hologram on the surface of chocolate bars. To do this he started with a metal shim, made from a conventional relief master hologram, and used it to stamp copies in the chocolate. He had to find the right chocolate rigidity to record and retain the patterns, as is also needed, but perhaps less difficult to achieve in making conventional plastic copies [27]. Of course, the image quality would suffer if the chocolate went through a long enough experience of high enough temperature or was pressed down upon or rubbed in handling. This market was revisited more recently and expanded to include hard candies in addition to chocolate [28].

#### **2.3 Photochromic materials**

Photochromic materials generally darken on exposure to light. They are usually responsive to blue or UV wavelengths. Their most distinctive characteristic is that the photoreaction taking place is completely reversible, so that they make possible materials for erasable holographic memories or repeat nondestructive testing using holographic interferometry. They have been demonstrated mostly in glass doped with silver halides. These materials have essentially unlimited resolution, as the photosensitive molecules are distributed evenly throughout, not as crystals or long-chain polymers or monomers. Against this advantage there is the problem that the image decays with time, lasting perhaps 10 minutes at room temperature, though it can be slowed down by refrigeration. They are also sensitive only in the blue and UV, and not to longer wavelengths. The exposure technique began with illumination by white light, which darkened the glass uniformly, then exposure to the laser light, which selectively bleached the glass where the exposure intensity was greatest. If left alone after that, the glass gradually lightened to transparency. The image could be speedcleared and readied for re-exposure by another white light illumination. In the 1960s I experimented using photochromic glass used for darkenable sunglasses, supplied by Corning, to make holograms. This worked in that I recorded holographic images, but the extreme insensitivity to red laser light forced 5-minute exposures with an unwidened 5 mW beam. The viewing of the reconstructed image, by laser light, will also bleach out the image, though that too could be slowed by refrigerating the sample.

#### **2.4 Dichromated gelatin emulsions**

Dichromated gelatin was one of the earliest alternatives developed to silver halide gelatin and gained favor because it recorded phase holograms directly by both very large index of refraction modulations, up in the range of 0.05, and strong surface distortions, yielding very high diffraction efficiencies and very bright images. The surface distortions can be copied onto metal that in turn could make the metal shim for stamping plastic copies. The internal transparency is excellent and the scattering noise is low. The majority of the "novelty" holograms, such as glasses with holographic eyes and keychain fobs, were mastered with or actually directly sold as sealed dichromates.

Saxby [29] gives reasonably detailed instructions for coating glass plates with DCG emulsion by three different methods:


Gelatin can be sensitized by being premixed with dichromate (ammonium dichromate, (NH4)2Cr2o7) or by dipping a gelatin-coated plate into a dichromate solution.

If plates are not pre-hardened before exposure, they will tend to dissolve during processing. But this is a somewhat delicate task, because if the emulsion is too hard, then the refractive index modulation and the diffraction efficiency will be suppressed, and also scattering will increase.

**29**

*2.5.1 Developers*

cm2

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

addition of dyes. Normal exposures will be up to 100+ mJ/cm<sup>2</sup>

down again even if the index modulation increases beyond 0.05.

ments, Richard Rallison, has a good reference list [30].

*2.4.1 A possible alternative DCG finishing technique*

488 nm. Dichromate can be sensitized reasonably well to longer wavelengths by the

the diffraction efficiency to 100%, but if exposure continues, saturation can drive it

Dichromated gelatin is normally processed by fixing, bathing in water, then drying by immersion in successively more concentrated baths of isopropyl alcohol in water, and finishing with 100% alcohol. After processing the emulsion must be sealed to prevent further contact with moisture, to preserve the image. This is a common practice if the image is intended for display, and not for copying. In the latter case, the surface relief is reproduced in a metal coating (a "shim") and used to

One of the pioneers of the use of DCG for display holograms and optical ele-

An alternative mode of finishing dichromated gelatin holograms might exist. In 1905 Lippmann commented [31], referring to his interference color photographic process, which works on the principle of a reflection hologram: "the question arose in my mind whether this transitory effect of moisture could not be permanently

"I soaked the plate in a solution of potassium iodide instead of in pure water. On drying, the colors were visible, though feebly so. The potassium iodide thus remained in the film, unequally distributed between maxima and minima of interference. On proceeding to pour over the dry iodized film a solution of silver nitrate (20%), the colors became extremely brilliant, and, on drying the plate, lost none of their striking character." The images were so bright that when viewed in transmission, instead of reflection, the complementary colors could be seen, and they were also brilliant. It remains to be seen whether images on emulsions processed like this

Silver halide emulsions are still the most favored variety for display holograms. This is because they can be easily sensitized across the visible spectrum and are more sensitive than any of the alternatives (typically requiring exposures of 1 mJ/

 or less), thus permitting the use of shorter exposures and lower power, less expensive lasers. They can also be processed to give low scattering noise and high diffraction efficiency. They are happy living unsealed in normal environments, in contrast to DCG emulsions, but less mechanically robust than photopolymers. The gold standard for consideration of silver halide gelatin techniques is still Bjelkhagen's 1993 book [9], followed closely by more recent texts [10, 32]. The complications and considerations are numerous, but the practice has encompassed a large range of materials and processing, and many approaches allow good control of results, including color control, low noise, and high diffraction efficiency.

There is a very long list of developers that have been tried for holography. The most successful ones have involved one or more of the historically important devel-

Development occurs in two basic ways, sometimes in combination. The first way involves reduction of the silver halide crystals that contain excited electrons created

opers such as metol, hydroquinone, pyrogallol, amidol, and ascorbic acid.

. It is possible to drive

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

stamp or injection mold the copies.

replaced by that of a solid, stable body."

remain stable or fade (blacken) due to printout.

**2.5 Silver halide gelatin emulsions**

The natural sensitivity of dichromated emulsion is to the blue and UV. As a result the laser that has been most used is likely the argon ion laser operating at

#### *Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

*Holographic Materials and Applications*

**2.4 Dichromated gelatin emulsions**

directly sold as sealed dichromates.

horizontally

the edges

DCG emulsion by three different methods:

pressed, and also scattering will increase.

1.Pouring along the top edge of a standing plate

Photochromic materials generally darken on exposure to light. They are usually responsive to blue or UV wavelengths. Their most distinctive characteristic is that the photoreaction taking place is completely reversible, so that they make possible materials for erasable holographic memories or repeat nondestructive testing using holographic interferometry. They have been demonstrated mostly in glass doped with silver halides. These materials have essentially unlimited resolution, as the photosensitive molecules are distributed evenly throughout, not as crystals or long-chain polymers or monomers. Against this advantage there is the problem that the image decays with time, lasting perhaps 10 minutes at room temperature, though it can be slowed down by refrigeration. They are also sensitive only in the blue and UV, and not to longer wavelengths. The exposure technique began with illumination by white light, which darkened the glass uniformly, then exposure to the laser light, which selectively bleached the glass where the exposure intensity was greatest. If left alone after that, the glass gradually lightened to transparency. The image could be speedcleared and readied for re-exposure by another white light illumination. In the 1960s I experimented using photochromic glass used for darkenable sunglasses, supplied by Corning, to make holograms. This worked in that I recorded holographic images, but the extreme insensitivity to red laser light forced 5-minute exposures with an unwidened 5 mW beam. The viewing of the reconstructed image, by laser light, will also bleach out the image, though that too could be slowed by refrigerating the sample.

Dichromated gelatin was one of the earliest alternatives developed to silver halide gelatin and gained favor because it recorded phase holograms directly by both very large index of refraction modulations, up in the range of 0.05, and strong surface distortions, yielding very high diffraction efficiencies and very bright images. The surface distortions can be copied onto metal that in turn could make the metal shim for stamping plastic copies. The internal transparency is excellent and the scattering noise is low. The majority of the "novelty" holograms, such as glasses with holographic eyes and keychain fobs, were mastered with or actually

Saxby [29] gives reasonably detailed instructions for coating glass plates with

3.Spin coating the plate while pouring the emulsion on it from its center out to

Gelatin can be sensitized by being premixed with dichromate (ammonium dichromate, (NH4)2Cr2o7) or by dipping a gelatin-coated plate into a dichromate solution. If plates are not pre-hardened before exposure, they will tend to dissolve during processing. But this is a somewhat delicate task, because if the emulsion is too hard, then the refractive index modulation and the diffraction efficiency will be sup-

The natural sensitivity of dichromated emulsion is to the blue and UV. As a result the laser that has been most used is likely the argon ion laser operating at

2.Using a Meyer Bar to draw the emulsion across the plate while it lies

**2.3 Photochromic materials**

**28**

488 nm. Dichromate can be sensitized reasonably well to longer wavelengths by the addition of dyes. Normal exposures will be up to 100+ mJ/cm<sup>2</sup> . It is possible to drive the diffraction efficiency to 100%, but if exposure continues, saturation can drive it down again even if the index modulation increases beyond 0.05.

Dichromated gelatin is normally processed by fixing, bathing in water, then drying by immersion in successively more concentrated baths of isopropyl alcohol in water, and finishing with 100% alcohol. After processing the emulsion must be sealed to prevent further contact with moisture, to preserve the image. This is a common practice if the image is intended for display, and not for copying. In the latter case, the surface relief is reproduced in a metal coating (a "shim") and used to stamp or injection mold the copies.

One of the pioneers of the use of DCG for display holograms and optical elements, Richard Rallison, has a good reference list [30].

#### *2.4.1 A possible alternative DCG finishing technique*

An alternative mode of finishing dichromated gelatin holograms might exist. In 1905 Lippmann commented [31], referring to his interference color photographic process, which works on the principle of a reflection hologram: "the question arose in my mind whether this transitory effect of moisture could not be permanently replaced by that of a solid, stable body."

"I soaked the plate in a solution of potassium iodide instead of in pure water. On drying, the colors were visible, though feebly so. The potassium iodide thus remained in the film, unequally distributed between maxima and minima of interference. On proceeding to pour over the dry iodized film a solution of silver nitrate (20%), the colors became extremely brilliant, and, on drying the plate, lost none of their striking character." The images were so bright that when viewed in transmission, instead of reflection, the complementary colors could be seen, and they were also brilliant. It remains to be seen whether images on emulsions processed like this remain stable or fade (blacken) due to printout.

#### **2.5 Silver halide gelatin emulsions**

Silver halide emulsions are still the most favored variety for display holograms. This is because they can be easily sensitized across the visible spectrum and are more sensitive than any of the alternatives (typically requiring exposures of 1 mJ/ cm2 or less), thus permitting the use of shorter exposures and lower power, less expensive lasers. They can also be processed to give low scattering noise and high diffraction efficiency. They are happy living unsealed in normal environments, in contrast to DCG emulsions, but less mechanically robust than photopolymers.

The gold standard for consideration of silver halide gelatin techniques is still Bjelkhagen's 1993 book [9], followed closely by more recent texts [10, 32]. The complications and considerations are numerous, but the practice has encompassed a large range of materials and processing, and many approaches allow good control of results, including color control, low noise, and high diffraction efficiency.

#### *2.5.1 Developers*

There is a very long list of developers that have been tried for holography. The most successful ones have involved one or more of the historically important developers such as metol, hydroquinone, pyrogallol, amidol, and ascorbic acid.

Development occurs in two basic ways, sometimes in combination. The first way involves reduction of the silver halide crystals that contain excited electrons created

by absorbing photons and that constitute the latent image specks. The developer reduces these to form metallic silver, and the specks agglomerate to form grains. This is termed chemical development and amplifies the creation speed of the silver grains by a factor over 100,000 times. The second way uses the developer to carry silver in solution to deposit on the latent specks. These form the latent image, and a fix is used to remove unexposed silver halide. The development continues to deposit more silver on the specks. This is a rather slow process but can result in very fine grains, often called colloidal, which can decrease the light scattering and photosensitivity and thus the proclivity to printout from further exposure to light. When development is complete, the result is an amplitude hologram.

The most widely used developer formula is the one formerly issued by Kodak as D-19. It has been widely used because it gives consistently good clarity and brightness with most emulsions, is easy to use and prepare, and is one of the safest of the possibilities. It is hydroquinone (8 g)-based, with metol (2 g), sodium sulfite (90 g), sodium carbonate (52.5 g), and potassium bromide (5 g) mixed in distilled water (1 liter). It also has excellent keeping qualities.

#### *2.5.2 Fixing*

It is conventional in photography to fix the developed image by dissolving all unexposed silver halide crystals, so that further exposure to light does not darken them. This is the role of "hypo," which was discovered by Sir John Herschel at the beginning of photography in 1839. However, many fixers (including that one) shrink the emulsion as they remove the halide crystals. Fixers are used in some silver halide holography practices, but for reflection holograms, this will create a color shift to the blue. This has led to strategies that omit fixer. Desensitizers can be used in place of fixers for transmission holograms [9].

#### *2.5.3 Bleaches*

To convert an amplitude hologram to a phase hologram, and thereby achieve diffraction efficiencies in the range of 70–90%, the usual tactic is to bleach the emulsion. There is a large range of bleaches too. They are principally of two types: rehalogenating and reversal. Reversal bleaches convert the silver grains to soluble complexes that are removed from the emulsion, leaving behind the unexposed silver halide crystals, which had not been removed by fixing. This leaves voids where the silver was removed and some shrinkage. For reflection holograms this leads to color shifts, as mentioned above, but has little effect on transmission holograms. Rehalogenating bleaches recombine the developed silver with the halides. If the emulsion was fixed first, this will also result in shrinkage and color shift from the fix step. If there is no fixing, this will leave the thickness unchanged. The image contrast will receive a big boost as the image becomes transparent and is converted to a phase hologram. The migration of the silver causes voids, and this leads to some scattering, which creates a milky haze that is exposure dependent. One of the various formulas that gives good results and is relatively safe is due to Ed Wesly. It consists of ferric sulfate (12 gms), disodium EDTA (12 gms), potassium bromide (30 gms), and sodium bisulfate (50 gms), to make up to 1 liter of distilled water. It keeps very well.

#### *2.5.4 Redevelopers*

The bleached emulsion can be put through developer again. One choice is to use a developer that is slow and yields smaller and more spherical silver grains, of the colloidal type referred to above. This will reduce the scattering caused by the bleach

**31**

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

*2.5.5 Hypersensitizers and color-shift treatments (pre-exposure)*

2.Pre-exposure soak in triethanolamine (TEA)

step and can yield clear images with excellent contrast and brightness. A very simple developer for this purpose can be made by dissolving 10 gms of ascorbic acid in 1 liter of distilled water. It has a long shelf life and is safe. Soaking the bleached plate in this solution while illuminating it with a white light source will gradually darken the plate to a brown-red tone, after which it can be rinsed and dried. If the white fog is still present, it can be redeveloped again. The smaller grains are also less

The above regime, of D-19 developer, ferric EDTA rehalogenating bleach, and ascorbic acid redeveloper, is simple and reliably gives good results for reflection and transmission holograms. Other combinations can give better results in some cases but are mostly more dangerous, and the solutions keep less well. Often, a very last step is a soak in a wetting agent, such as Photo-Flo, which can add protection

There are several methods that have been developed to hypersensitize silver halide emulsions. All three enhance sensitivity without enlarging grain size or

The first two are long known to holographers. The third was recently published. Of the first two, a simple dip of 30 seconds into distilled water followed by drying will enhance sensitivity by about a factor of 2. This effect has a shelf life of perhaps a day. The pre-exposure dip into TEA solutions followed by drying likely has a shelf life of several months. It also achieves a speed gain of about a factor of 2, without affecting resolution, though careful technique is needed if streak-free

TEA is well known to holographers primarily for its ability to shift the color of the final result in reflection holograms. This occurs because TEA swells the emulsion. The normal interference layered fringe pattern, with spacing of ½ the laser wavelength, is recorded. Then, when the TEA is rinsed out, before development, the emulsion shrinks back, and after development and drying, the layer spacing shrinks to yield image reconstruction at a shorter wavelength. The amount of swelling effect is controlled by a combination of dip time and TEA concentration in water. The range of colors accessible by this technique is from the laser color to the

There is also a post-development color-shift treatment by soaking the emulsion in a swelling agent, frequently the chemical sorbitol. This will shift the color from

The third type of hypersensitization treatment was recently published [33]. It builds on an invention by Belloni and her coworkers that employs a pre-exposure soak in a solution of formate in water. Tests showed that a dilution of 1/100 molar would give the desired effect and leave no residue on the emulsion. The emulsion is soaked for 30 seconds to a minute and then dried. It is then exposed and developed normally. However, there is a peculiarity to the way the effect works. The speed gain is maximized by delaying development by 10–15 minutes after the exposure. With that delay, the speed gain is 10×s. If the development is done immediately

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

prone to printout.

against printout.

results are desired.

bluest visible color.

the laser color to colors to the red of that.

reducing resolution. These are:

1.Pre-exposure soak in water

3.Pre-exposure soak in formate

#### *Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

step and can yield clear images with excellent contrast and brightness. A very simple developer for this purpose can be made by dissolving 10 gms of ascorbic acid in 1 liter of distilled water. It has a long shelf life and is safe. Soaking the bleached plate in this solution while illuminating it with a white light source will gradually darken the plate to a brown-red tone, after which it can be rinsed and dried. If the white fog is still present, it can be redeveloped again. The smaller grains are also less prone to printout.

The above regime, of D-19 developer, ferric EDTA rehalogenating bleach, and ascorbic acid redeveloper, is simple and reliably gives good results for reflection and transmission holograms. Other combinations can give better results in some cases but are mostly more dangerous, and the solutions keep less well. Often, a very last step is a soak in a wetting agent, such as Photo-Flo, which can add protection against printout.

#### *2.5.5 Hypersensitizers and color-shift treatments (pre-exposure)*

There are several methods that have been developed to hypersensitize silver halide emulsions. All three enhance sensitivity without enlarging grain size or reducing resolution. These are:

1.Pre-exposure soak in water

*Holographic Materials and Applications*

by absorbing photons and that constitute the latent image specks. The developer reduces these to form metallic silver, and the specks agglomerate to form grains. This is termed chemical development and amplifies the creation speed of the silver grains by a factor over 100,000 times. The second way uses the developer to carry silver in solution to deposit on the latent specks. These form the latent image, and a fix is used to remove unexposed silver halide. The development continues to deposit more silver on the specks. This is a rather slow process but can result in very fine grains, often called colloidal, which can decrease the light scattering and photosensitivity and thus the proclivity to printout from further exposure to light. When

The most widely used developer formula is the one formerly issued by Kodak as D-19. It has been widely used because it gives consistently good clarity and brightness with most emulsions, is easy to use and prepare, and is one of the safest of the possibilities. It is hydroquinone (8 g)-based, with metol (2 g), sodium sulfite (90 g), sodium carbonate (52.5 g), and potassium bromide (5 g) mixed in distilled water

It is conventional in photography to fix the developed image by dissolving all unexposed silver halide crystals, so that further exposure to light does not darken them. This is the role of "hypo," which was discovered by Sir John Herschel at the beginning of photography in 1839. However, many fixers (including that one) shrink the emulsion as they remove the halide crystals. Fixers are used in some silver halide holography practices, but for reflection holograms, this will create a color shift to the blue. This has led to strategies that omit fixer. Desensitizers can be

To convert an amplitude hologram to a phase hologram, and thereby achieve diffraction efficiencies in the range of 70–90%, the usual tactic is to bleach the emulsion. There is a large range of bleaches too. They are principally of two types: rehalogenating and reversal. Reversal bleaches convert the silver grains to soluble complexes that are removed from the emulsion, leaving behind the unexposed silver halide crystals, which had not been removed by fixing. This leaves voids where the silver was removed and some shrinkage. For reflection holograms this leads to color shifts, as mentioned above, but has little effect on transmission holograms. Rehalogenating bleaches recombine the developed silver with the halides. If the emulsion was fixed first, this will also result in shrinkage and color shift from the fix step. If there is no fixing, this will leave the thickness unchanged. The image contrast will receive a big boost as the image becomes transparent and is converted to a phase hologram. The migration of the silver causes voids, and this leads to some scattering, which creates a milky haze that is exposure dependent. One of the various formulas that gives good results and is relatively safe is due to Ed Wesly. It consists of ferric sulfate (12 gms), disodium EDTA (12 gms), potassium bromide (30 gms), and sodium bisulfate (50 gms), to make up to 1 liter of distilled water. It keeps very well.

The bleached emulsion can be put through developer again. One choice is to use a developer that is slow and yields smaller and more spherical silver grains, of the colloidal type referred to above. This will reduce the scattering caused by the bleach

development is complete, the result is an amplitude hologram.

(1 liter). It also has excellent keeping qualities.

used in place of fixers for transmission holograms [9].

*2.5.2 Fixing*

*2.5.3 Bleaches*

**30**

*2.5.4 Redevelopers*


The first two are long known to holographers. The third was recently published. Of the first two, a simple dip of 30 seconds into distilled water followed by drying will enhance sensitivity by about a factor of 2. This effect has a shelf life of perhaps a day. The pre-exposure dip into TEA solutions followed by drying likely has a shelf life of several months. It also achieves a speed gain of about a factor of 2, without affecting resolution, though careful technique is needed if streak-free results are desired.

TEA is well known to holographers primarily for its ability to shift the color of the final result in reflection holograms. This occurs because TEA swells the emulsion. The normal interference layered fringe pattern, with spacing of ½ the laser wavelength, is recorded. Then, when the TEA is rinsed out, before development, the emulsion shrinks back, and after development and drying, the layer spacing shrinks to yield image reconstruction at a shorter wavelength. The amount of swelling effect is controlled by a combination of dip time and TEA concentration in water. The range of colors accessible by this technique is from the laser color to the bluest visible color.

There is also a post-development color-shift treatment by soaking the emulsion in a swelling agent, frequently the chemical sorbitol. This will shift the color from the laser color to colors to the red of that.

The third type of hypersensitization treatment was recently published [33]. It builds on an invention by Belloni and her coworkers that employs a pre-exposure soak in a solution of formate in water. Tests showed that a dilution of 1/100 molar would give the desired effect and leave no residue on the emulsion. The emulsion is soaked for 30 seconds to a minute and then dried. It is then exposed and developed normally. However, there is a peculiarity to the way the effect works. The speed gain is maximized by delaying development by 10–15 minutes after the exposure. With that delay, the speed gain is 10×s. If the development is done immediately

after exposure, the speed gain is only a factor of 5. This converts emulsions of silver halide from requiring three or four photons per latent silver halide molecule to needing only one, a quantum efficiency of 1. There is a very slight color shift to the red, which means there was a very slight swelling of the emulsion due to the treatment.

This hypersensitizer was tested only on one holographic emulsion (Slavich PFG-01), with one developer (D-19), and for a continuous exposure, though it should in theory work for all other silver halide emulsions. It was not tested in conjunction with a pulsed laser exposure. There is evidence that latent images made by pulsed exposures fade more rapidly than by continuous exposure [9]. This leaves to open a question: will the formate pretreatment be less effective for pulsed exposures due to their tendency to rapid fading, or will it be more effective as it will slow down the rate of fading? This remains to be tested.

#### **3. Conclusions**

In this review we have looked at a range of processes and media for making highquality holograms and some of the associated history.

The last 50 years has seen the market for holograms grow in certain commercial sectors. These have exerted pressure to develop master holographic emulsions for mass market uses such as security markers and HOEs. There have been many experiments tried, and recently two photopolymers, one by Bayer and one by Liti, have appeared. They have improved stability, improved resistance to shrinkage and color shifts, and in the case of Liti are completely self-developing, in analogy to the very old photographic printing-out processes.

Metamaterials appear to be in the process of revolutionizing holography, as well as optics in general. There will be a great deal of work done in order to commercialize them, and their primary application is likely to be for HOEs.

Display holography has not seen the market growth of other applications, despite efforts by artists and entrepreneurs, and there have not been big resources devoted to them. The most recent advance is a method of presoaking silver halide emulsions with a formate solution, which hypersensitizes them by a factor of 5–10×'s. This can allow shorter exposures, making vibration less of an issue, or allow the making of larger area images using lasers of modest power output. It can be possible to have similar effects for a wide range of photochemistries.

**33**

**Author details**

William Alschuler

provided the original work is properly cited.

California Institute of the Arts, Valencia, California, USA

\*Address all correspondence to: walschuler@hotmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

#### **Conflict of interest**

The author declares no conflict of interest.

*Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

*Holographic Materials and Applications*

rate of fading? This remains to be tested.

quality holograms and some of the associated history.

very old photographic printing-out processes.

The author declares no conflict of interest.

ize them, and their primary application is likely to be for HOEs.

possible to have similar effects for a wide range of photochemistries.

treatment.

**3. Conclusions**

**Conflict of interest**

after exposure, the speed gain is only a factor of 5. This converts emulsions of silver halide from requiring three or four photons per latent silver halide molecule to needing only one, a quantum efficiency of 1. There is a very slight color shift to the red, which means there was a very slight swelling of the emulsion due to the

This hypersensitizer was tested only on one holographic emulsion (Slavich PFG-01), with one developer (D-19), and for a continuous exposure, though it should in theory work for all other silver halide emulsions. It was not tested in conjunction with a pulsed laser exposure. There is evidence that latent images made by pulsed exposures fade more rapidly than by continuous exposure [9]. This leaves to open a question: will the formate pretreatment be less effective for pulsed exposures due to their tendency to rapid fading, or will it be more effective as it will slow down the

In this review we have looked at a range of processes and media for making high-

The last 50 years has seen the market for holograms grow in certain commercial

Metamaterials appear to be in the process of revolutionizing holography, as well as optics in general. There will be a great deal of work done in order to commercial-

Display holography has not seen the market growth of other applications, despite efforts by artists and entrepreneurs, and there have not been big resources devoted to them. The most recent advance is a method of presoaking silver halide emulsions with a formate solution, which hypersensitizes them by a factor of 5–10×'s. This can allow shorter exposures, making vibration less of an issue, or allow the making of larger area images using lasers of modest power output. It can be

sectors. These have exerted pressure to develop master holographic emulsions for mass market uses such as security markers and HOEs. There have been many experiments tried, and recently two photopolymers, one by Bayer and one by Liti, have appeared. They have improved stability, improved resistance to shrinkage and color shifts, and in the case of Liti are completely self-developing, in analogy to the

**32**

#### **Author details**

William Alschuler California Institute of the Arts, Valencia, California, USA

\*Address all correspondence to: walschuler@hotmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Gabor D. A new microscopic principle. Nature. 1948;**161**:777-778

[2] Leith E, Upatnieks J. New techniques in wavefront reconstruction. Journal of the Optical Society of America A. 1961;**51**:1469

[3] Leith E, Upatnieks J. Photography by laser. Scientific American. 1965;**212**(6):24-36

[4] Lippmann G. La photographie des couleurs. *Comptes Rendus de l'Académie des Sciences*. 1891;**112**(5):274-275

[5] Lippmann G. Sur la theorie de la photographie des couleurs simples et composees par la methode interferentielle. Comptes Rendus de l'Academie des Sciences. 1894;**118**(3):92-102

[6] History of Metamaterials. Available at: https://en.wikipedia.org/wiki/ History\_of\_metamaterials [Accessed: February 2019]

[7] Meta Group, Cummer Group, Jokerst Group, and Mikkelsen Group. Holographic Metasurfaces. 2013. Available at: https://metamaterials.duke. edu/research/holographic-metasurfaces [Accessed: February 2019]

[8] Bourzac K. Stretchy Holograms made from a metamaterial. May 10, 2017. Available at: https://cen.acs.org/content/ cen/articles/95/web/2017/05/Stretchyholograms-made-metamaterial.html [Accessed: February 2019]

[9] Bjelkhagen H. Silver Halide Emulsions for Holography and Their Processing. New York: Springer Verlag; 1993

[10] Saxby G. Practical Holography. 3rd ed. Boca Raton: CRC Press; 2003. pp. 318-322 and Appendix 5. DOI 13:978-1-4200-366-3 (e-book-PDF)

[11] Kim N. Holographic applications Based on photopolymer materials. International Workshop on Photonics and Applications, Hanoi, Vietnam, April 5-8, 2004. Available at: https:// pdfs.semanticscholar.org/326d/2fa8a6 00f15bd172bb2825bb1915b854aba8.pdf [Accessed: February 2019]

[12] Lin SH, et al. Exposure schedule for multiplexing holograms in photopolymer. In: SPIE Conference on Photorefractive Fiber and Crystal Devices: Materials, Optical Properties and Applications V, Denver, Colorado; July 1999; SPIE. Vol. 3801

[13] John FP, and seq. Dupont Photopolymer. July 25, 2006. Available at: https://holowiki.org/forum/ viewtopic.php?f=30&t=5722&sid=4 7e38831d8f7af17eb5c67a0affb7aa6&st art=10

[14] NA. Handling procedures for dupont photopolymer films. Tech. Bulletin TB-944 (revised 03/08). Available at: http:// www.dupont.com/content/dam/ dupont/products-and-services/ electronic-and-electrical-materials/ printed-circuit-board-materials/ documents/DEC-Handing-Procedures-Photopolymer-Films.pdf [Accessed: February 2019]

[15] Orlic S, Eichler HJ. Optical data storage using holographic gratings. In: Driessen A, editor. Nonlinear Optics for the Information Society Proceedings of the Third Annual COST Action P2. Boston: Kluwer Academic Publishers; 2001. pp. 79-86. DOI: 10.1007/978/94-015-1267-1

[16] Bruder F-K, Flacke T, and Rolle T. The Chemistry and Physics of Bayfol® HX Film Holographic Photopolymer. September 26, 2017. Available at: https:// polymers-09-00472-v2.pdf [Accessed: February 2019]

**35**

2019]

2019]

*Emulsions, Photochemistry, and Processing Factors for Display Holograms*

April 2, 1997. Available at: https:// www.nytimes.com/1997/04/02/garden/ where-no-candy-has-gone-beforelight-as-the-secret-ingredient.html

[28] Doss HM. Holograms: From Credit Cards to Chocolate. August 5, 2014. Available at: http://www.physicscentral. com/explore/action/holograms.cfm

[29] Saxby G. Practical Holography. 3rd ed. Boca Raton: CRC Press; 2003. pp. 323-324 and Appendix 5. DOI 13:978-1-4200-366-3 (e-book-PDF)

[30] Rallison R. The History of

pdf [Accessed: February 2019]

1905;**140**:1508-1509

2800-7 (e-book-PDF)

2019]

[33] Alschuler WR. Formate as a sensitivity enhancer of holographic emulsions. In: Proceedings Volume 10558, Practical Holography XXXII: Displays, Materials, and Applications. Vol. 105580C. 2018. doi: 10.1117/12.2286668 [Accessed February

[31] Lippmann G. Photographs in colors from negatives by the Lippmann process. British Journal of Photography. June 30, 1905: 505 and in the Comptes Rendu de l'Academie des Sciences (CR).

Dichromates. 1996. Available at: https:// wasatchphotonics.com/wp-content/ uploads/The-History-of-Dichromates1.

[32] Bjelkhagen H, Brotherton-Ratcliffe D. Ultra-Realistic Imaging: Advanced Techniques in Analog and Digital Colour Holography. Boca Raton: CRC Press; 2013. pp. 59-60. DOI: 13:978-1-4-

[Accessed: February 2019]

[Accessed: February 2019]

*DOI: http://dx.doi.org/10.5772/intechopen.85753*

[17] NA. https://www.films.covestro. com/~/media/Product%20Center/ FF/Documents/Brochures/Bayfol%20 HX%20-%20Holography.ashx [Accessed: February 2019]

[18] N. A. Liti Hologram Kit. 2019. Available at: http://www.litiholo.com/

[19] Krakow G. How to Make Holograms at Home. May 6, 2005. Available at: http://www.nbcnews.com/id/7759505/ ns/technology\_and\_science-tech\_ and\_gadgets/t/how-make-holograms-

hologram-kits.html

home/#.XGgr2Px7m7M

Available at: http://www.

[Accessed: February 2019]

February, 2019]

2001;**8**(4):241-244

[20] NA. Liti Commercial Images.

[21] NA. Liti Film Specifications. Available at: https://litiholo.com/ Litiholo%20CRT20%20hologram%20 film%20spec%20sheet.pdf [Accessed:

[22] Kasai I et al. A practical seethrough head mounted display using a holographic element. Optical Review.

de Physique. 1908;**7**:821-825

[23] Lippmann G. Epreuves reversible donnant la sensation du relief. Journal

[24] Jang C et al. Recent progress in seethrough three-dimensional displays using holographic optical elements [Invited]. Applied Optics. 2016;**55**(3):A71-A85

[25] Hess R. (Bob). 2004. Available at: https://holowiki.org/forum/viewtopic. php?f=30&t=2929 [Accessed: February

[26] NA. Digilens Available at: https:// www.digilens.com/ [Accessed: February

[27] Hilts PJ. Where No Candy Has Gone Before: Light as the Secret Ingredient.

litiholographics.com/commercial.html

*Emulsions, Photochemistry, and Processing Factors for Display Holograms DOI: http://dx.doi.org/10.5772/intechopen.85753*

[17] NA. https://www.films.covestro. com/~/media/Product%20Center/ FF/Documents/Brochures/Bayfol%20 HX%20-%20Holography.ashx [Accessed: February 2019]

[18] N. A. Liti Hologram Kit. 2019. Available at: http://www.litiholo.com/ hologram-kits.html

[19] Krakow G. How to Make Holograms at Home. May 6, 2005. Available at: http://www.nbcnews.com/id/7759505/ ns/technology\_and\_science-tech\_ and\_gadgets/t/how-make-hologramshome/#.XGgr2Px7m7M

[20] NA. Liti Commercial Images. Available at: http://www. litiholographics.com/commercial.html [Accessed: February 2019]

[21] NA. Liti Film Specifications. Available at: https://litiholo.com/ Litiholo%20CRT20%20hologram%20 film%20spec%20sheet.pdf [Accessed: February, 2019]

[22] Kasai I et al. A practical seethrough head mounted display using a holographic element. Optical Review. 2001;**8**(4):241-244

[23] Lippmann G. Epreuves reversible donnant la sensation du relief. Journal de Physique. 1908;**7**:821-825

[24] Jang C et al. Recent progress in seethrough three-dimensional displays using holographic optical elements [Invited]. Applied Optics. 2016;**55**(3):A71-A85

[25] Hess R. (Bob). 2004. Available at: https://holowiki.org/forum/viewtopic. php?f=30&t=2929 [Accessed: February 2019]

[26] NA. Digilens Available at: https:// www.digilens.com/ [Accessed: February 2019]

[27] Hilts PJ. Where No Candy Has Gone Before: Light as the Secret Ingredient.

April 2, 1997. Available at: https:// www.nytimes.com/1997/04/02/garden/ where-no-candy-has-gone-beforelight-as-the-secret-ingredient.html [Accessed: February 2019]

[28] Doss HM. Holograms: From Credit Cards to Chocolate. August 5, 2014. Available at: http://www.physicscentral. com/explore/action/holograms.cfm [Accessed: February 2019]

[29] Saxby G. Practical Holography. 3rd ed. Boca Raton: CRC Press; 2003. pp. 323-324 and Appendix 5. DOI 13:978-1-4200-366-3 (e-book-PDF)

[30] Rallison R. The History of Dichromates. 1996. Available at: https:// wasatchphotonics.com/wp-content/ uploads/The-History-of-Dichromates1. pdf [Accessed: February 2019]

[31] Lippmann G. Photographs in colors from negatives by the Lippmann process. British Journal of Photography. June 30, 1905: 505 and in the Comptes Rendu de l'Academie des Sciences (CR). 1905;**140**:1508-1509

[32] Bjelkhagen H, Brotherton-Ratcliffe D. Ultra-Realistic Imaging: Advanced Techniques in Analog and Digital Colour Holography. Boca Raton: CRC Press; 2013. pp. 59-60. DOI: 13:978-1-4- 2800-7 (e-book-PDF)

[33] Alschuler WR. Formate as a sensitivity enhancer of holographic emulsions. In: Proceedings Volume 10558, Practical Holography XXXII: Displays, Materials, and Applications. Vol. 105580C. 2018. doi: 10.1117/12.2286668 [Accessed February 2019]

**34**

1993

*Holographic Materials and Applications*

[1] Gabor D. A new microscopic principle. Nature. 1948;**161**:777-778

1961;**51**:1469

**References**

1965;**212**(6):24-36

1894;**118**(3):92-102

February 2019]

[2] Leith E, Upatnieks J. New techniques in wavefront reconstruction. Journal of the Optical Society of America A.

[11] Kim N. Holographic applications Based on photopolymer materials. International Workshop on Photonics and Applications, Hanoi, Vietnam, April 5-8, 2004. Available at: https:// pdfs.semanticscholar.org/326d/2fa8a6 00f15bd172bb2825bb1915b854aba8.pdf

[12] Lin SH, et al. Exposure schedule for multiplexing holograms in photopolymer. In: SPIE Conference on Photorefractive Fiber and Crystal Devices: Materials, Optical Properties and Applications V, Denver, Colorado;

[Accessed: February 2019]

July 1999; SPIE. Vol. 3801

art=10

February 2019]

February 2019]

[13] John FP, and seq. Dupont

at: https://holowiki.org/forum/ viewtopic.php?f=30&t=5722&sid=4 7e38831d8f7af17eb5c67a0affb7aa6&st

[14] NA. Handling procedures for dupont photopolymer films. Tech. Bulletin TB-944 (revised 03/08). Available at: http:// www.dupont.com/content/dam/ dupont/products-and-services/ electronic-and-electrical-materials/ printed-circuit-board-materials/ documents/DEC-Handing-Procedures-Photopolymer-Films.pdf [Accessed:

[15] Orlic S, Eichler HJ. Optical data storage using holographic gratings. In: Driessen A, editor. Nonlinear Optics for the Information Society Proceedings of the Third Annual COST Action P2. Boston: Kluwer Academic Publishers; 2001. pp. 79-86. DOI: 10.1007/978/94-015-1267-1

[16] Bruder F-K, Flacke T, and Rolle T. The Chemistry and Physics of Bayfol® HX Film Holographic Photopolymer. September 26, 2017. Available at: https:// polymers-09-00472-v2.pdf [Accessed:

Photopolymer. July 25, 2006. Available

[3] Leith E, Upatnieks J. Photography

[4] Lippmann G. La photographie des couleurs. *Comptes Rendus de l'Académie* 

[6] History of Metamaterials. Available at: https://en.wikipedia.org/wiki/ History\_of\_metamaterials [Accessed:

Available at: https://metamaterials.duke. edu/research/holographic-metasurfaces

[8] Bourzac K. Stretchy Holograms made from a metamaterial. May 10, 2017. Available at: https://cen.acs.org/content/ cen/articles/95/web/2017/05/Stretchyholograms-made-metamaterial.html

[7] Meta Group, Cummer Group, Jokerst Group, and Mikkelsen Group. Holographic Metasurfaces. 2013.

[Accessed: February 2019]

[Accessed: February 2019]

[9] Bjelkhagen H. Silver Halide Emulsions for Holography and Their Processing. New York: Springer Verlag;

[10] Saxby G. Practical Holography. 3rd ed. Boca Raton: CRC Press; 2003. pp. 318-322 and Appendix 5. DOI 13:978-1-4200-366-3 (e-book-PDF)

*des Sciences*. 1891;**112**(5):274-275

[5] Lippmann G. Sur la theorie de la photographie des couleurs simples et composees par la methode interferentielle. Comptes Rendus de l'Academie des Sciences.

by laser. Scientific American.

**37**

Section 2

Optical Systems
