**3. Study of volume light-sensitive media using recorded holograms**

Volume recording media of about a millimeter thickness for holography exhibit, as a rule, very small values of modulation amplitude of optical parameters (Δn ≈ 10-3÷10-5) and small spatial size (fractions of a micron) of behavior of physico-chemical processes, taking place under impact of radiation and post-exposure treatment. These particular properties make it difficult to use standard instruments and procedures in study of such media and their changes under impact of radiation and other factors. The basic technique for research into volume RM parameters is the holographic one – investigation of media through construction of holograms and study of their parameters. The technique comprises taskoriented alteration of controlled variables in recording and post-exposure treatment of holograms; measurement of hologram parameters at each stage of their construction; theoretical analysis, enabling to correlate the changes of hologram parameters with experimental conditions and RM parameters.

As a test object, we used hologram-gratings, which were theoretically analyzed with the help of coupled wave theory (Kogelnik, 1969). This theory is an indispensable tool in study of RM using recorded holograms, as it allows relating the measured hologram parameters with the amplitude of modulation of optical characteristics of a medium.

The work dealt with phase and amplitude-phase transmission hologram-gratings with different thicknesses and constants, constructed, as a rule, in a symmetrical (or close to that) configuration of interfering beams relative to the sample surface. Main measured holographic parameters were the diffraction efficiency (DE) and selectivity contour of holograms, constructed under different recording and post-exposure treatment conditions.

Diffraction efficiency (η) of a hologram was defined as the ratio of diffracted beam intensity (Id) to the sum of beam intensities behind the hologram: η = Id /(Id + I0), where I0 is the intensity of the zero-order diffraction beam. Maximum values of DE for a hologram are attained at reconstruction under Bragg conditions. According to the coupled wave theory, the DE of a volume phase hologram-grating at reconstruction under Bragg conditions can be represented by the formulas: η = sin2φ1 for transmission holograms and η = tanh2 φ1 reflection holograms. Phase modulation amplitude, φ1, is defined by the expression:

$$\mathbf{q}\_1 = \pi \mathbf{n}\_1 \mathbf{T} / \lambda \cos \theta \tag{1}$$

where n1 is the amplitude of variation of the first harmonic of refractive index of the medium; Т is the hologram thickness; λ is the radiation wavelength; 2θ is the angle of diffracted beam (Id) to zero beam (I0). This equations relate the measured parameter of transmission hologram (η) to RM parameters (n1, T) and experimental conditions (λ, cos θ). Fig. 1а shows dependences of DE on the magnitude of phase modulation for phase transmission (curve 1) and reflection (curve 2) holograms in the variation range of phase modulation of media under study (0 < φ1 < 2π).

2010). Attempts to implement the principle using other compositions was less successful. However, experts consider the principle today among the most promising in creation of volume recording materials for holographic memory (Ashley et al., 2000; Shelby, 2002; Liu

Volume recording media of about a millimeter thickness for holography exhibit, as a rule, very small values of modulation amplitude of optical parameters (Δn ≈ 10-3÷10-5) and small spatial size (fractions of a micron) of behavior of physico-chemical processes, taking place under impact of radiation and post-exposure treatment. These particular properties make it difficult to use standard instruments and procedures in study of such media and their changes under impact of radiation and other factors. The basic technique for research into volume RM parameters is the holographic one – investigation of media through construction of holograms and study of their parameters. The technique comprises taskoriented alteration of controlled variables in recording and post-exposure treatment of holograms; measurement of hologram parameters at each stage of their construction; theoretical analysis, enabling to correlate the changes of hologram parameters with

As a test object, we used hologram-gratings, which were theoretically analyzed with the help of coupled wave theory (Kogelnik, 1969). This theory is an indispensable tool in study of RM using recorded holograms, as it allows relating the measured hologram parameters

The work dealt with phase and amplitude-phase transmission hologram-gratings with different thicknesses and constants, constructed, as a rule, in a symmetrical (or close to that) configuration of interfering beams relative to the sample surface. Main measured holographic parameters were the diffraction efficiency (DE) and selectivity contour of holograms, constructed under different recording and post-exposure treatment

Diffraction efficiency (η) of a hologram was defined as the ratio of diffracted beam intensity (Id) to the sum of beam intensities behind the hologram: η = Id /(Id + I0), where I0 is the intensity of the zero-order diffraction beam. Maximum values of DE for a hologram are attained at reconstruction under Bragg conditions. According to the coupled wave theory, the DE of a volume phase hologram-grating at reconstruction under Bragg conditions can be represented by the formulas: η = sin2φ1 for transmission holograms and η = tanh2 φ1 reflection holograms. Phase modulation amplitude, φ1, is defined by the

where n1 is the amplitude of variation of the first harmonic of refractive index of the

diffracted beam (Id) to zero beam (I0). This equations relate the measured parameter of transmission hologram (η) to RM parameters (n1, T) and experimental conditions (λ, cos θ). Fig. 1а shows dependences of DE on the magnitude of phase modulation for phase transmission (curve 1) and reflection (curve 2) holograms in the variation range of phase

λ

φ1 = πn1Т/λcos θ (1)

is the radiation wavelength; 2θ is the angle of

with the amplitude of modulation of optical characteristics of a medium.

**3. Study of volume light-sensitive media using recorded holograms** 

experimental conditions and RM parameters.

medium; Т is the hologram thickness;

modulation of media under study (0 < φ1 < 2π).

at al., 2010).

conditions.

expression:

In the presence of the amplitude component, the DE of an amplitude-phase transmission hologram is defined taking into account that the modulation amplitude of the first harmonic of hologram absorption index (α1) and the average hologram absorption index (α0). Fig. 1a presents the dependence (curve 3) for the case α1 = α0 = 0.05. The oscillatory nature of dependence η(φ1) is seen to persist. The data on Fig. 1a characterize variation of DE of holograms at their reconstruction under Bragg conditions. To describe the deviations from Bragg conditions, mismatch parameter (ξ) is used. Dependence of hologram DE, or diffracted beam intensity, on mismatch parameter ξ is the selectivity contour of a hologram. One recognizes the spectral selectivity contour of a hologram – dependence Id(λ) at θ = θBr and the angular selectivity contour of a hologram – Id (θ) at λ = λBr. The halfwidths of spectral and angular selectivity contours, Δλ and Δθ, are a measure of hologram selectivity. Comparison of selectivity of hologram-gratings with different DE by measured values of Δλ and Δθ is appropriately done in the variation range of phase modulation of a medium 0.1π < φ1 < 0.5π, where the halfwidth values for a selectivity contour are practically independent of the DE of a hologram and are defined by its constant and thickness.

As have been noted, the dependence of DE of transmission volume holograms on phase modulation value (φ1) is of oscillatory nature. Here, for phase holograms without absorption, φ1 = kπ ± sin-1 √η, where *k* = 0, 1, 2, 3; therefore, different sections of DE variation range (as indicated on Fig. 1a) are to correspond to different formulas for calculation of φ1 by measured values of η. Fig. 1b shows selectivity contours of transmission phase holograms with DE = 50% (at reconstruction under Bragg conditions), which were obtained at different values of phase modulation amplitude. It is clearly seen that φ1 for high-efficiency transmission holograms can be unambiguously found by DE values only with account of selectivity contour shape as opposed to thinlayer holograms, where, as a rule, φ1 < 0.5π. The study of amplitude-phase holograms should involve not only the hologram selectivity contour (the dependence of diffracted beam intensity on parameter ξ), but also the dependence of intensity of zero beam that passed the hologram without changing the direction, on parameter ξ (Fig. 1c).

Dependences Id(ξ) and I0(ξ), given on Fig. 1 (b,c) for hologram-gratings with different values of phase modulation, display unsymmetrical nature of dependence I0(ξ) of amplitude-phase holograms (Fig. 1c) as opposed to phase holograms (Fig. 1b). Thus, symmetry of dependence I0(ξ) with respect to Bragg conditions is evidence of absence of the amplitude component in the hologram under study, which is necessary for correctness of calculations and estimation.

When the coupled wave theory is used to analyze experimental results, account is to be taken of the measurements of hologram parameters taking place, as a rule, in the air, and the formula-defined relationship of the studied parameters being established inside the medium. Comparison of experimentally measured hologram parameters to those calculated theoretically for given experimental conditions allows finding the amplitude of modulation of optical parameters of the medium in a hologram and their variation in processes under study as well as estimating some other characteristics, e. g., the uniformity of the lightsensitive agent distribution in the sample bulk, the effective hologram thickness against the geometric dimensions of the sample and so on.

Light-Sensitive Media-Composites for

high-resolution RM.

in all visible region (see Fig.2, curves 2).

Recording Volume Holograms Based on Porous Glass and Polymer 51

pores with an immersion liquid, a 1 mm thick sample exhibits high transparence practically

The light sensitivity of porous silver-containing recording medium is provided by the silverhalide component with gelatin as a protective colloid. Light-sensitive component is formed as a solid-phase shell that is rigidly bound with framework walls (see Fig.2с,d), while central regions of internal framework cavities remain unfilled, thus forming a network of through capillaries, which provides access for liquid and gaseous chemical agents. The synthesis of light-sensitive composite is performed directly inside a sample with the use of KBr, KJ, AgNO3 and gelatin solution. The synthesis process is typical for manufacturing

The solid-phase shell occupies less than 10 % of the total volume of an air-dry sample. Under impregnation of water solutions, the shell swells and fills the entire internal pore volume without changing its localization relative to framework walls, to which it is rigidly bound. In synthesis of AgHal, the size of formed particles and the variation of their localization in the course of post-exposure treatment cannot exceed the maximum size of porous ducts, which amounts when using matrices NPG-17 to 20 nm. The feature makes AgHal-porous glass (AgHal-PG) media fundamentally distinct from AgHal film plates, where it is practically impossible to create an ensemble of particles with limitation of maximum sizes and to ensure their rigid localization during the hologram construction.

Fig. 2. Spectral dependence of transmission (a) and optical density (b) of porous samples in air (curves 1, 3) and in water immersion (curves 2, 4); 1, 2 – a sample NPG-17 of thickness 1 mm; 3, 4 – a sample with gelatin shell of internal capillaries: measurements in the air are relative to the air; those in water are relative the water. Schematic drawing of the cross-

Unsensitized samples of medium-composite are sensitive in the intrinsic light sensitivity of AgHal in spectral region λ < 510 nm. The developed synthesis process allows performing optical sensitization of AgHal composite with the use of dyes for the visible and near IR regions. During experiments, hologram recording with the use of Ar-ion laser (488 nm) was

section of porous glass (c) and light-sensitive medium AgHal-PG (d).

Fig. 1. а - dependence of diffraction efficiency (η) on phase modulation amplitude (φ1) for volume phase transmission (curve 1) and reflection (curve 2) holograms; amplitude-phase transmission hologram with absorption index γ0 = γ1 = 0.1 (curve 3). b,c - intensity distribution in diffracted (solid lines) and zero (dotted lines) beams at deviation from Bragg conditions (ξ) at reconstruction of transmission phase hologram (b) and transmission amplitude-phase hologram (c) at γ1 = γ0 = 0.1 with phase modulation: 1 – φ1 = 0.25π, 2 – φ1 = 0.75π, 3 – φ1 = 1.25π, 4 – φ1 = 1.75π.

#### **4. Light-sensitive AgHal-porous glass medium-composite**

#### **4.1 AgHal-porous glass samples: Manufacture and main features**

The rigid framework of porous volume media provides porous glass, obtained from two-phase glass by treating samples in acid and alkali solution. The need to secure the requirements to rigid matrix during development of recording media led to creation of samples NPG-17 with stable and reproducible parameters, which are obtained from initial two-phase glass DV-1 by the developed technology. Samples NPG-17, used to create RM, are polished plates or disks 1-3 mm thick. The average pore diameter is 17 nm, the poreoccupied free volume of a sample is (52 ± 4) %. The light absorption in the short-wave region by air-dry samples is quite considerable because of scattering by porous structure and absorption by framework components (see Fig.2, curves 1). Under impregnation of

Fig. 1. а - dependence of diffraction efficiency (η) on phase modulation amplitude (φ1) for volume phase transmission (curve 1) and reflection (curve 2) holograms; amplitude-phase

distribution in diffracted (solid lines) and zero (dotted lines) beams at deviation from Bragg conditions (ξ) at reconstruction of transmission phase hologram (b) and transmission amplitude-phase hologram (c) at γ1 = γ0 = 0.1 with phase modulation: 1 – φ1 = 0.25π, 2 –

The rigid framework of porous volume media provides porous glass, obtained from two-phase glass by treating samples in acid and alkali solution. The need to secure the requirements to rigid matrix during development of recording media led to creation of samples NPG-17 with stable and reproducible parameters, which are obtained from initial two-phase glass DV-1 by the developed technology. Samples NPG-17, used to create RM, are polished plates or disks 1-3 mm thick. The average pore diameter is 17 nm, the poreoccupied free volume of a sample is (52 ± 4) %. The light absorption in the short-wave region by air-dry samples is quite considerable because of scattering by porous structure and absorption by framework components (see Fig.2, curves 1). Under impregnation of

transmission hologram with absorption index γ0 = γ1 = 0.1 (curve 3). b,c - intensity

**4. Light-sensitive AgHal-porous glass medium-composite 4.1 AgHal-porous glass samples: Manufacture and main features** 

φ1 = 0.75π, 3 – φ1 = 1.25π, 4 – φ1 = 1.75π.

pores with an immersion liquid, a 1 mm thick sample exhibits high transparence practically in all visible region (see Fig.2, curves 2).

The light sensitivity of porous silver-containing recording medium is provided by the silverhalide component with gelatin as a protective colloid. Light-sensitive component is formed as a solid-phase shell that is rigidly bound with framework walls (see Fig.2с,d), while central regions of internal framework cavities remain unfilled, thus forming a network of through capillaries, which provides access for liquid and gaseous chemical agents. The synthesis of light-sensitive composite is performed directly inside a sample with the use of KBr, KJ, AgNO3 and gelatin solution. The synthesis process is typical for manufacturing high-resolution RM.

The solid-phase shell occupies less than 10 % of the total volume of an air-dry sample. Under impregnation of water solutions, the shell swells and fills the entire internal pore volume without changing its localization relative to framework walls, to which it is rigidly bound. In synthesis of AgHal, the size of formed particles and the variation of their localization in the course of post-exposure treatment cannot exceed the maximum size of porous ducts, which amounts when using matrices NPG-17 to 20 nm. The feature makes AgHal-porous glass (AgHal-PG) media fundamentally distinct from AgHal film plates, where it is practically impossible to create an ensemble of particles with limitation of maximum sizes and to ensure their rigid localization during the hologram construction.

Fig. 2. Spectral dependence of transmission (a) and optical density (b) of porous samples in air (curves 1, 3) and in water immersion (curves 2, 4); 1, 2 – a sample NPG-17 of thickness 1 mm; 3, 4 – a sample with gelatin shell of internal capillaries: measurements in the air are relative to the air; those in water are relative the water. Schematic drawing of the crosssection of porous glass (c) and light-sensitive medium AgHal-PG (d).

Unsensitized samples of medium-composite are sensitive in the intrinsic light sensitivity of AgHal in spectral region λ < 510 nm. The developed synthesis process allows performing optical sensitization of AgHal composite with the use of dyes for the visible and near IR regions. During experiments, hologram recording with the use of Ar-ion laser (488 nm) was

Light-Sensitive Media-Composites for

temperature.

Recording Volume Holograms Based on Porous Glass and Polymer 53

modification of developer PRG-1 was proposed, which was used in the performed experiments: anhydrous sodium sulfite – 0.2 g, hydroquinone – 0.2 g, KBr – 0.15 g, water – 100 ml. Development time is 8-20 hrs at temperature 20 °C. Stop bath is dipping in 0.2 % solution of citric acid for 30 min. Rinsing is in distilled water for 20 hrs. Drying is at room

Developed particles of AgHal-porous glass medium-composite, like those of AgHal film plates, represent particles of recovered metallic silver of colloid structure. Fig. 4a presents attenuation spectra of developed film plates PFG-03 (curve 1) and AgHal-PG (curve 2) after development to formulation PRG-1 so as to obtain developed silver particles of colloid structure. Curve 3 represents attenuation spectrum of diluted water preparation of Ag-PG

As seen from the given results, all attenuation spectra are of highly marked selective nature with a maximum in the short-wave part of the visible region (0.39÷0.43 μm), which is evidence of colloid structure of studied particles, close in shape to a sphere. When comparing spectra, a shift of the maximum of attenuation spectra of Ag-PG samples to the short-wave region is clearly seen, which is evidence of reduced average developed particle

Fig. 4. a - attenuation spectra of AgHal media after the development stage with creation of particles of colloid structure (curves 1-3) and after the bleaching stage (curve 4): 1 – film plate PFG-03); 2 – AgHal-PG samples with thickness 0.3 mm; 3 – preparation of dispersed sample 3, diluted with water; 4 – AgHal-PG sample after the bleaching stage. b – phase modulation amplitude φ1 of PG-holograms as function of the filler of the free volume of pore nf for λ = 0.63 μm (solid curves) and λ = 1.5 μm (dashed curves): AgHal-PG after development (1,2) and after the bleaching (3); porous glass with dichromated gelatin (4).

The development process results are affected by a lot of factors, related to the conditions of performing the synthesis of light-sensitive composite and hologram recording, i. e. "life history" of samples prior to their development. The angular selectivity contours holograms on Fig. 3b (curve 1) demonstrate the influence – of immense interest is the possibility to obtain a developed hologram contour, having no sidelobes. Angular selectivity contours of developed holograms (see Fig. 3a, curve 2) have as a rule higher sidelobes, which is

sample of type 2, which is dispersed (powdered) and spread in water solution.

size in AgHal-PG as compared to film plates PFG-03 .

assured by intrinsic sensitivity of AgHal and that with the use of He-Ne laser (633 nm) was due to optical sensitization. Hologram parameters were measured in the red (633-655 nm) and near IR regions (1.5 μm).

#### **4.2 Basic stages of hologram construction process on samples of AgHal-porous glass**

The stages of hologram construction on samples of AgHal-PG medium are typical for holograms on traditional AgHal media: pre-exposure treatment of samples (e. g., impregnation of an immersion into a sample to reduce scattering); exposure; development; additional post-exposure treatment - stop bath, fixation, bleaching and so on; rinsing; drying. The network of through capillaries allows using water solutions for pre- and postexposure treatment of porous samples, but essentially retards and modifies the physicochemical processes, developed to construct holograms on traditional AgHal materials.

Recording. During hologram recording in a medium under effect of radiation, formation of the so-called "latent image", typical for AgHal media, takes place. Latent image centers (LIC) cause practically no changes in optical properties of the sample and form a hologram that has, immediately after recording, low diffraction efficiency (< 1%). LICs are as a rule also the development centers that determine the formation of developed particles. Fig. 3a (curve 1) shows angular selectivity contours of "latent image" hologram. The width of measured contours is in agreement with the theoretical values, found for a hologram with constant-depth amplitude modulation, but the sidelobes exceed the calculated values. The fact is evidence that the LIC concentration near the sample surface is somewhat higher than on average in the sample.

Fig. 3. Angular selectivity contours of holograms at different stages of their construction: a a hologram at the latent image stage (curve 1), after development (curve 2), after thickness reduction owing to grinding away of surface layers of the sample (curve 3); b – developed hologram (curve 1) after the bleaching stage (curve 2) in air-dry condition (solid curves) and in water immersion (dashed curves).

Development. The basis of the elaborated process of development was taken to be that of construction of holograms on film plates with the use of developer PRG-1. The elaboration of the process of hologram construction took account of special features of the elaborated process of synthesis of light-sensitive component in nano-porous matrix and minimized the effect of the chemical activity of quartz-like framework on silver recovery process. A

assured by intrinsic sensitivity of AgHal and that with the use of He-Ne laser (633 nm) was due to optical sensitization. Hologram parameters were measured in the red (633-655 nm)

The stages of hologram construction on samples of AgHal-PG medium are typical for holograms on traditional AgHal media: pre-exposure treatment of samples (e. g., impregnation of an immersion into a sample to reduce scattering); exposure; development; additional post-exposure treatment - stop bath, fixation, bleaching and so on; rinsing; drying. The network of through capillaries allows using water solutions for pre- and postexposure treatment of porous samples, but essentially retards and modifies the physicochemical processes, developed to construct holograms on traditional AgHal materials. Recording. During hologram recording in a medium under effect of radiation, formation of the so-called "latent image", typical for AgHal media, takes place. Latent image centers (LIC) cause practically no changes in optical properties of the sample and form a hologram that has, immediately after recording, low diffraction efficiency (< 1%). LICs are as a rule also the development centers that determine the formation of developed particles. Fig. 3a (curve 1) shows angular selectivity contours of "latent image" hologram. The width of measured contours is in agreement with the theoretical values, found for a hologram with constant-depth amplitude modulation, but the sidelobes exceed the calculated values. The fact is evidence that the LIC concentration near the sample surface is somewhat higher than

Fig. 3. Angular selectivity contours of holograms at different stages of their construction: a a hologram at the latent image stage (curve 1), after development (curve 2), after thickness reduction owing to grinding away of surface layers of the sample (curve 3); b – developed hologram (curve 1) after the bleaching stage (curve 2) in air-dry condition (solid curves) and

Development. The basis of the elaborated process of development was taken to be that of construction of holograms on film plates with the use of developer PRG-1. The elaboration of the process of hologram construction took account of special features of the elaborated process of synthesis of light-sensitive component in nano-porous matrix and minimized the effect of the chemical activity of quartz-like framework on silver recovery process. A

**4.2 Basic stages of hologram construction process on samples of AgHal-porous** 

and near IR regions (1.5 μm).

on average in the sample.

in water immersion (dashed curves).

**glass** 

modification of developer PRG-1 was proposed, which was used in the performed experiments: anhydrous sodium sulfite – 0.2 g, hydroquinone – 0.2 g, KBr – 0.15 g, water – 100 ml. Development time is 8-20 hrs at temperature 20 °C. Stop bath is dipping in 0.2 % solution of citric acid for 30 min. Rinsing is in distilled water for 20 hrs. Drying is at room temperature.

Developed particles of AgHal-porous glass medium-composite, like those of AgHal film plates, represent particles of recovered metallic silver of colloid structure. Fig. 4a presents attenuation spectra of developed film plates PFG-03 (curve 1) and AgHal-PG (curve 2) after development to formulation PRG-1 so as to obtain developed silver particles of colloid structure. Curve 3 represents attenuation spectrum of diluted water preparation of Ag-PG sample of type 2, which is dispersed (powdered) and spread in water solution.

As seen from the given results, all attenuation spectra are of highly marked selective nature with a maximum in the short-wave part of the visible region (0.39÷0.43 μm), which is evidence of colloid structure of studied particles, close in shape to a sphere. When comparing spectra, a shift of the maximum of attenuation spectra of Ag-PG samples to the short-wave region is clearly seen, which is evidence of reduced average developed particle size in AgHal-PG as compared to film plates PFG-03 .

Fig. 4. a - attenuation spectra of AgHal media after the development stage with creation of particles of colloid structure (curves 1-3) and after the bleaching stage (curve 4): 1 – film plate PFG-03); 2 – AgHal-PG samples with thickness 0.3 mm; 3 – preparation of dispersed sample 3, diluted with water; 4 – AgHal-PG sample after the bleaching stage. b – phase modulation amplitude φ1 of PG-holograms as function of the filler of the free volume of pore nf for λ = 0.63 μm (solid curves) and λ = 1.5 μm (dashed curves): AgHal-PG after development (1,2) and after the bleaching (3); porous glass with dichromated gelatin (4).

The development process results are affected by a lot of factors, related to the conditions of performing the synthesis of light-sensitive composite and hologram recording, i. e. "life history" of samples prior to their development. The angular selectivity contours holograms on Fig. 3b (curve 1) demonstrate the influence – of immense interest is the possibility to obtain a developed hologram contour, having no sidelobes. Angular selectivity contours of developed holograms (see Fig. 3a, curve 2) have as a rule higher sidelobes, which is

Light-Sensitive Media-Composites for

for parameters of Ag-PG holograms.

Recording Volume Holograms Based on Porous Glass and Polymer 55

Fig. 5. a - theoretically found spectral dependences of optical constants of porous model medium, due to the presence of "ideal" silver particles (l = 8 nm) and "real" particles (l ≈ 2 nm); γ and Δn are, respectively, absorption constant and refractive index. b – dependence of phase incursion (ΔnT/λ), due to the presence of silver particles in the medium, on silver coverage, C, of the studied sample: experimental measurements with the

The dependence of diffraction efficiency of transmission amplitude-phase hologram gratings, formed by colloid silver particles, on silver concentration, given on Fig. 6, is of oscillatory nature because of presence of strong phase modulation and is dependent on the operating wavelength and optical properties of its constituent silver particles. The given data show the maximum values of DE of transmission AgPG holograms to be attained at a certain value of silver concentration: as the wavelength grows, so do both the maximum values of DE and the silver concentration needed to attain them. The DE values for holograms in a porous medium with ideal particles can be thought of as a theoretical limit

Fig. 6. Theoretically calculated dependences of DE of transmission Ag-PG holograms on developed silver concentration CAg for porous medium 1 mm thick with ideal (l = 8 nm, solid curves) and real (l ≈ 2 nm, dashed curves) particles: λ = 0.63 μm (a) and λ = 1.5 μm (b). The presence of direct proportionality of refractive index of developed RM, containing colloid silver particles, on developed silver concentration, is confirmed by measurements of

use of developed holography film plates PFG-02 (dots); calculations by measured attenuation spectra with the use of dispersion Kramers-Kronig relations (crosses).

evidence of increasing non-uniformity of hologram modulation amplitude across the sample depth in the course of development. Grinding away of surface layers of the developed sample allows constructing a hologram less thick, but more uniform in distribution of modulation amplitude (Fig. 3a, curve 3).

Bleaching. The bleaching process, when colloid particles of metallic silver are transformed to those of silver halide, transparent in the visible region (see Fig. 4a, curve 4), changes also the hologram structure due to changes of optical parameters of the components and their distribution across the sample depth, which is manifested in alteration of the selectivity contour of the bleached hologram as compared to the initial developed hologram (Fig. 3b, curves 2). Impregnation of an immersion into free volume of pores of bleached holograms causes their DE to lower, which is to be kept in mind as the use of bleached holograms without impregnation of immersion is often impossible because of noticeable scattering as compared to unbleached ones.

#### **4.3 Investigation of developed samples and Ag-porous glass holograms**

Developed Ag-PG holograms are amplitude-phase ones: they exhibit absorption in the visible region, especially high in its short-wave part, which strongly affects the potential of usage and application of such holograms. The presence of amplitude modulation is evidenced by non-symmetric dependence of I0(δθ) on conditions of hologram reconstruction, namely, I0(-δθ) ≠ I0(+δθ) at λ = 0.63 μm. The amplitude modulation decreases noticeably with growing wavelength of reconstructing radiation (see Fig. 4b) and at λ = 1.5 μm the Ag-PG hologram can be considered a purely phase one, which is evidenced by experimentally measured I0(δθ) and Id(δθ): dependence I0(δθ) is symmetric and typical for a phase hologram, I0(-δθ) = I0(+δθ).

The relation between amplitude and phase components in such holograms is determined by spectral dependence of permittivity (ε) of developed silver particles, which is in the case of finely dispersed particles different from the corresponding dependences for bulk silver. This was the reason for performing theoretical estimates and calculations of spectral dependences of effective optical parameters of porous model medium, containing Raleigh silver particles, and parameters of constructed Ag-PG holograms (Sukhanov et al., 1996).

According to calculations based on using the concept of limitation of free path length of electron in a small particle (Kreibig, 1970, 1978, 1981), composite medium has the minimum width of attenuation band at the size 15-20 nm of its constituent silver particles that have no defects of crystalline structure. Free path length (l) of electron in a particle of such ideal medium is l = 8 nm. The value of free path of electron in silver particles, composing a real medium with attenuation spectrum shown on Fig. 4 (curve 3), is found to be l = 2 nm.

This free path value is not an estimate of the average particle size, as in addition to size there are other unmanageable factors: contamination, crystalline structure defects etc. But it is quantity l that determines the effective optical constants of a medium, which are used in further calculations to estimate the parameters of AgPG holograms. Comparison of effective optical parameters of "ideal" (with particles l = 8 nm) and real (with particles l = 2 nm) media with identical structure parameters, whose calculated values are given on Fig. 5a, shows the attenuation spectrum of real medium to be essentially wider than that of the "ideal" one, whereas the change of refractive index due to the presence of silver particles (ΔnAg) is practically the same and is determined by their concentration, СAg, (the quoted calculations used the volume silver concentration in the medium equal to 10-4).

evidence of increasing non-uniformity of hologram modulation amplitude across the sample depth in the course of development. Grinding away of surface layers of the developed sample allows constructing a hologram less thick, but more uniform in distribution of

Bleaching. The bleaching process, when colloid particles of metallic silver are transformed to those of silver halide, transparent in the visible region (see Fig. 4a, curve 4), changes also the hologram structure due to changes of optical parameters of the components and their distribution across the sample depth, which is manifested in alteration of the selectivity contour of the bleached hologram as compared to the initial developed hologram (Fig. 3b, curves 2). Impregnation of an immersion into free volume of pores of bleached holograms causes their DE to lower, which is to be kept in mind as the use of bleached holograms without impregnation of immersion is often impossible because of noticeable scattering as

Developed Ag-PG holograms are amplitude-phase ones: they exhibit absorption in the visible region, especially high in its short-wave part, which strongly affects the potential of usage and application of such holograms. The presence of amplitude modulation is evidenced by non-symmetric dependence of I0(δθ) on conditions of hologram reconstruction, namely, I0(-δθ) ≠ I0(+δθ) at λ = 0.63 μm. The amplitude modulation decreases noticeably with growing wavelength of reconstructing radiation (see Fig. 4b) and at λ = 1.5 μm the Ag-PG hologram can be considered a purely phase one, which is evidenced by experimentally measured I0(δθ) and Id(δθ): dependence I0(δθ) is symmetric and typical for a

The relation between amplitude and phase components in such holograms is determined by spectral dependence of permittivity (ε) of developed silver particles, which is in the case of finely dispersed particles different from the corresponding dependences for bulk silver. This was the reason for performing theoretical estimates and calculations of spectral dependences of effective optical parameters of porous model medium, containing Raleigh silver particles, and parameters of constructed Ag-PG holograms (Sukhanov et al., 1996). According to calculations based on using the concept of limitation of free path length of electron in a small particle (Kreibig, 1970, 1978, 1981), composite medium has the minimum width of attenuation band at the size 15-20 nm of its constituent silver particles that have no defects of crystalline structure. Free path length (l) of electron in a particle of such ideal medium is l = 8 nm. The value of free path of electron in silver particles, composing a real medium with attenuation spectrum shown on Fig. 4 (curve 3), is found to be l = 2 nm. This free path value is not an estimate of the average particle size, as in addition to size there are other unmanageable factors: contamination, crystalline structure defects etc. But it is quantity l that determines the effective optical constants of a medium, which are used in further calculations to estimate the parameters of AgPG holograms. Comparison of effective optical parameters of "ideal" (with particles l = 8 nm) and real (with particles l = 2 nm) media with identical structure parameters, whose calculated values are given on Fig. 5a, shows the attenuation spectrum of real medium to be essentially wider than that of the "ideal" one, whereas the change of refractive index due to the presence of silver particles (ΔnAg) is practically the same and is determined by their concentration, СAg, (the quoted

**4.3 Investigation of developed samples and Ag-porous glass holograms** 

calculations used the volume silver concentration in the medium equal to 10-4).

modulation amplitude (Fig. 3a, curve 3).

compared to unbleached ones.

phase hologram, I0(-δθ) = I0(+δθ).

Fig. 5. a - theoretically found spectral dependences of optical constants of porous model medium, due to the presence of "ideal" silver particles (l = 8 nm) and "real" particles (l ≈ 2 nm); γ and Δn are, respectively, absorption constant and refractive index. b – dependence of phase incursion (ΔnT/λ), due to the presence of silver particles in the medium, on silver coverage, C, of the studied sample: experimental measurements with the use of developed holography film plates PFG-02 (dots); calculations by measured attenuation spectra with the use of dispersion Kramers-Kronig relations (crosses).

The dependence of diffraction efficiency of transmission amplitude-phase hologram gratings, formed by colloid silver particles, on silver concentration, given on Fig. 6, is of oscillatory nature because of presence of strong phase modulation and is dependent on the operating wavelength and optical properties of its constituent silver particles. The given data show the maximum values of DE of transmission AgPG holograms to be attained at a certain value of silver concentration: as the wavelength grows, so do both the maximum values of DE and the silver concentration needed to attain them. The DE values for holograms in a porous medium with ideal particles can be thought of as a theoretical limit for parameters of Ag-PG holograms.

Fig. 6. Theoretically calculated dependences of DE of transmission Ag-PG holograms on developed silver concentration CAg for porous medium 1 mm thick with ideal (l = 8 nm, solid curves) and real (l ≈ 2 nm, dashed curves) particles: λ = 0.63 μm (a) and λ = 1.5 μm (b).

The presence of direct proportionality of refractive index of developed RM, containing colloid silver particles, on developed silver concentration, is confirmed by measurements of

Light-Sensitive Media-Composites for

at exposure 10-2 J/cm2.

**5. Polymeric material Difphen** 

**5.1 The principle of action** 

introduced.

possible in the course of post-exposure treatment.

obtained with the use of bleaching or dichromated gelatin.

of PQ-based polymeric medium with diffusion enhancement.

according to schematic diagram (Chercasov et al., 1991):

hν RH R●

respectively, the polymer molecule and radical.

particles that form a finished hologram is several times their diameter.

Recording Volume Holograms Based on Porous Glass and Polymer 57

3. Light sensitivity of unsensitized samples across the spectrum is limited by intrinsic sensitivity of AgHal (λ<510 nm) and amounts to 10-3 J/cm2 when recording holograms at 488 nm with DE = 10 %. The possibility to perform optical sensitization of AgHal-PG was demonstrated and samples, sensitive in the red region, were obtained: holograms with DE = 50 % were constructed when recording by He-Ne laser radiation (λ = 633 nm)

4. Formation of low-efficiency holograms (DE <1 %) during recording leaves the structure of recorded interference pattern undistorted, and substantial hologram enhancement is

5. Post-exposure treatment is performed with the use of traditional photochemical water solutions owing to the presence of through capillary network. Formulation can be modified, conditions of running the processes can be optimized, and new stages can be

6. Impregnation of an immersion into free volume of pores to reduce light scattering by porous samples is possible both at stages of recording and operation of constructed holograms. When using holograms formed by colloid silver particles, impregnation into free pore volume of an immersion with refractive index close to that of the framework (water, acetone, alcohol, CCl4) produces practically no change in their phase modulation (the change lies within the experimental error), as distinct from holograms,

7. Concentration of developed silver in the samples is close on order of magnitude to the corresponding parameter in AgHal film plates (1÷5 g/m2). The distance between

There are at present several modifications of polymeric light-sensitive media on the base of phenanthrenequinone (PQ), which implement the diffusion enhancement principle. The authors devised the technology to obtain a material, whose samples have certain holographic and physical-mechanical parameters, conditioned by modes of sample synthesis and hologram construction. The name Difphen (from words DIFfusion and PHENanthrenequinone) allows singling given material out of variety of other modifications

Samples of material Difphen (like some other materials of given group) represent a solid solution of organic dye PQ, uniformly distributed in polymethylmethacrylate (PMMA). Light sensitivity of the material results from capacity of PQ to bond to polymer under irradiation, transforming into 9,10-disubstituted derivative of phenanthrene (НPQR)

where 3PQ is the triplet-excited PQ molecule, НPQ● is the semiquinone radical, RH и R● are,

Samples of recording medium 1÷5 mm thick are obtained by means of bulk polymerization of PQ solutions in methylmethacrylate (MMA) between molding glass plates. Fig. 7a

PQ → 3PQ <sup>→</sup> <sup>Н</sup>PQ● <sup>→</sup> <sup>Н</sup>PQR (3)

refractive index of developed high-resolution AgHal materials (film plates PFG-02) in the red region λ ~ 650 nm. Experimental measurements, their results given on Fig. 5b, were performed under different conditions of formation of developed particles and at different developed silver concentrations; the resulting dependence can be represented by the empirical formula:

$$
\Delta \mathbf{n}\_{\text{Ag}} \mathbf{T}/\lambda = 0.28 \,\mathrm{C} \,\mathrm{(g/m^2)}\tag{2}
$$

where ΔnAg is the change of the medium refractive index due to the presence of silver particles; T is the medium thickness; C is the surface silver concentration (silver coverage in g/m2), measured by rhodanine method. Eq. (2) enables estimation of developed silver concentration by measured values of phase incursions in the red region λ ~ 650 nm due to the presence of colloid silver particles, without refinement of their optical parameters.

Experimental measurements of a hologram at λ = 1.5 μm are performed in the absence of the amplitude component and allow finding the phase modulation of a hologram at given wavelength, which allows estimating the spectral dependence of amplitude and phase modulation of the studied Ag-PG hologram, φ1(λ) and γ1(λ), by using known wavelength dependence of optical constants of real medium. Calculations of angular selectivity contours of amplitude-phase holograms at 0.63 μm, carried out by experimentally found phase modulation at λ = 1.5 μm, have shown satisfactory agreement with the results of experimental measurements.

This situation enables using known value φ1 (0.63-0.65 μm) to estimate the concentration of silver, forming given hologram, with the help of empirical dependence Eq. (2). The performed calculation have shown the volume concentration of silver, forming the Ag-PG hologram, to be C1 = (1÷3) 10-4, which corresponds to surface concentration 1÷5 g/m2, comparable to a similar value in developed holographic film plates PFG-02 and PFG-03. It should be noted that average hologram absorption γ0 (hence average silver concentration in the sample, C0) exceed the estimates found by values γ1 and C1 due to the presence of veil, formed by colloid silver particles, which take no part in hologram construction, yet lead to an increase of sample absorption in the spectral region of the absorption band of colloid particles.

#### **4.4 Distinctive features of AgHal-porous glass medium-composite in hologram construction**

The proposed version of light-sensitive porous medium-composite on the AgHal base significantly enhances the potential of using traditional AgHal media in holography, which is evidenced by its main features.

