**5. Holographic properties of PTR glasses**

Since the main purpose of the material is to serve as a holographic media, study of the refractive index dynamic range was held by utilizing holographic technique. For this purpose, Bragg gratings with period of 775 nm were recorded with UV radiation of He-Cd laser (λ = 325 nm). Conditions of thermal treatment as well as exposure schedule differed depending on glass type, due to the difference in the mechanism responsible for refractive index change. After UV exposure and thermal treatment, gratings were measured and analyzed using Collier [31] and Carretero [32] equations. All measurements were performed at the wavelength of He-Ne laser (*λ* = 632.8 nm). Analysis was made with respect to the form of the angular dependence contour either in the zero order or in the first-order of diffraction. Even though the gratings are quasisinusoidal, we confine our analysis and therefore material characterization with first harmonic of the refractive index modulation amplitude (RIMA). In this chapter, we will show the exposure dependencies of the RIMA for each glass and its behavior connected with thermal treatment schedule.

#### **5.1. Fluoride PTR glass**

In **Figure 13**, typical dependence of the RIMA on the UV exposure for our fluoride PTR glass is shown. One can see that there is a quite wide range of the exposures in which the RIMA is at maximum. Therefore, we assume that an optimum exposure for this glass lies within 0.4– 0.65 J/cm2 . Decreasing the RIMA with an increase in the exposure, we explain by the effect of the stray scattering of neutral silver clusters that appears during the recording process. This scattering affects the contrast of the interference pattern, thus lowering the RIMA in the grating.

**Figure 13.** Dependence of the RIMA on exposure.

nonirradiated bromide PTR glasses with bromine concentration <1 mol% does not change their refractive index [13]. On the contrary, the UV irradiation and subsequent heat treatment of bromide PTR glasses result in a significant increase in their refractive index. For the maximum bromine concentration, a difference *Δn* between the refractive indices of the UV-irradiated and

**Figure 12.** Effect of bromine concentration on the refractive index (*nd*) of PTR glass. 1—untreated glass samples, 2 glass samples after the heat treatment, 3—glass samples after the UV irradiation and subsequent heat treatment.

Since the main purpose of the material is to serve as a holographic media, study of the refractive index dynamic range was held by utilizing holographic technique. For this purpose, Bragg gratings with period of 775 nm were recorded with UV radiation of He-Cd laser (λ = 325 nm). Conditions of thermal treatment as well as exposure schedule differed depending on glass type, due to the difference in the mechanism responsible for refractive index change. After UV exposure and thermal treatment, gratings were measured and analyzed using Collier [31] and Carretero [32] equations. All measurements were performed at the wavelength of He-Ne laser (*λ* = 632.8 nm). Analysis was made with respect to the form of the angular dependence contour either in the zero order or in the first-order of diffraction. Even though the gratings are quasisinusoidal, we confine our analysis and therefore material characterization with first harmonic of the refractive index modulation amplitude (RIMA). In this chapter, we will show the exposure dependencies of the RIMA for each glass and its behavior connected with thermal

In **Figure 13**, typical dependence of the RIMA on the UV exposure for our fluoride PTR glass is shown. One can see that there is a quite wide range of the exposures in which the RIMA is

**5. Holographic properties of PTR glasses**

446 Holographic Materials and Optical Systems

treatment schedule.

**5.1. Fluoride PTR glass**

nonirradiated glasses after the heat treatment reaches magnitudes up to 0.8 × 10−3.

In **Figure 14**, the dependence of RIMA value on the duration of the thermal treatment is shown. One can see that this dependence is not linear. Basically, we can vary the duration together with temperature and obtain the same effect. For example, the RIMA value of 1.3 × 10−3 can be achieved for 8 h of thermal treatment at temperature of 500 C or for 130 h of heat treatment at temperature of 470°C.

**Figure 14.** Refractive index modulation amplitude of the grating in the fluorine PTR glass with respect to duration of the thermal treatment.

Additional studies of glass chemical composition allowed for implementing the complex optimization of components, the main goal being to decrease optical losses in the visible spectral range caused by the absorption band of colloidal silver [12]. Components that had undergone the concentration optimization were as follows: halides (fluorides and bromides) responsible for the growth of microcrystalline shell and crystalline phase; antimony that plays a key role in capturing and donating the photoelectrons arising upon the irradiation of cerium and subsequent thermal treatment of PTR glass; also, the concentration of impurities capable of capturing photoelectrons was lowered. As a result, a number of parameters were improved, thus exceeding those of commercially produced material. First of all, the problem of undesirable absorption in the visible spectral range was solved, which resulted in a great decrease in the induced optical losses caused by colloidal silver. PTR glass with the renewed composition shows, after the FTI crystallization process, no absorption band related to the colloidal particles in the optical loss spectra of a sample with a hologram recorded (**Figure 15**).

**Figure 15.** Absorption coefficient spectra of modified PTR glass with a hologram recorded.

**Figure 16.** Microscope image of the grating (a) right after the UV exposure and (b) after the heat treatment, and TEM image of the grating fringe (c).

Nowadays, the maximum RIMA for the fluoride PTR glass can be as high as 1.5 ×10−3. If we neglect scattering by the crystalline phase inside the glass, the maximum RIMA magnitude can be even greater (like 2.5 × 10−3).

Also, we performed the visualization of the recorded gratings right after an exposure with the UV radiation (**Figure 16(a)**) and after the heat treatment (**Figure 16(b)**). In **Figure 16(a)**, one can see the luminescence of the silver clusters in accord with the interference pattern, whereas, in **Figure 16(b)**, there is the grating itself only formed with NaF crystals.

#### **5.2. Chloride PTR glass**

responsible for the growth of microcrystalline shell and crystalline phase; antimony that plays a key role in capturing and donating the photoelectrons arising upon the irradiation of cerium and subsequent thermal treatment of PTR glass; also, the concentration of impurities capable of capturing photoelectrons was lowered. As a result, a number of parameters were improved, thus exceeding those of commercially produced material. First of all, the problem of undesirable absorption in the visible spectral range was solved, which resulted in a great decrease in the induced optical losses caused by colloidal silver. PTR glass with the renewed composition shows, after the FTI crystallization process, no absorption band related to the colloidal particles

in the optical loss spectra of a sample with a hologram recorded (**Figure 15**).

**Figure 15.** Absorption coefficient spectra of modified PTR glass with a hologram recorded.

image of the grating fringe (c).

448 Holographic Materials and Optical Systems

can be even greater (like 2.5 × 10−3).

**Figure 16.** Microscope image of the grating (a) right after the UV exposure and (b) after the heat treatment, and TEM

Nowadays, the maximum RIMA for the fluoride PTR glass can be as high as 1.5 ×10−3. If we neglect scattering by the crystalline phase inside the glass, the maximum RIMA magnitude

Also, we performed the visualization of the recorded gratings right after an exposure with the UV radiation (**Figure 16(a)**) and after the heat treatment (**Figure 16(b)**). In **Figure 16(a)**, one We only started studying the holographic characteristics of this type of PTR glasses. By now, investigations carried out were intended, by analogy with similar studies performed earlier for fluoride glasses, to estimate the maximum possible changes in the RIMA for these glasses.

**Figure 17.** Image of the sample with gratings recorded.

**Figure 18.** Example of the angular response of the mixed absorption—the phase holographic grating recorded in chloride PTR glass.

Gratings recoded on this glass are colored (**Figure 17**), which is why it is natural to assume the modulation of absorption in the grating. The first measurements performed on these gratings proved the validity of this assumption. As one can see in **Figure 18**, the angular response from the grating, indeed, has a poor symmetry. So far measurements were only at a single wavelength of 632.8 nm that is far enough from the resonance band of silver nanoparticles. Hence, the value of the absorption index modulation amplitude (AIMA) was expected to be rather small.

**Figure 18** shows the approximation of experimentally obtained angular response in zero order with a theoretical curve. The position of the central maximum of the latter is shifted, and the positions of side lobes are perfectly fitted to the experimental curve. Differences in the intensities of the side lobes are connected with scattering in the sample that is inflicted by silver nanoparticles. It is also clear that this grating has a strong RIMA because the shape of the contour includes a lot of side lobes and actually lacks the central maximum.

On the other hand, our theoretical analysis shows that the AIMA is quite weak compared with other materials. The reason can be due to the fact that the measurements are conducted in a region lying far enough from the main resonance band of the silver nanoparticles. For instance, AIMA for a sample subjected to the thermal treatment for 30 h and exposure of 4 J/cm2 is found to be almost 6 cm−1, which consists 85% of the total value of the absorption coefficient at this wavelength (7.12 cm−1). The fact that AIMA is a bit less than the latter can be explained in two ways. First, the occurrence of scattering during the recording process might create clusters outside the interference pattern, hence, lowering the contrast. Second, as seen in figure below, glass is colored even outside the irradiated region (pale red color); this can be also the reason for an additional increase in the absorption coefficient that is not connected with AIMA.

**Figure 19.** Typical dependence of the RIMA on exposure for chloride PTR glass.

According to this fact, one can expect the total absorption coefficient to be modulated in the region of resonance band, which can lead to really great AIMA magnitudes. In **Figure 19**, a typical dependence of the RIMA on exposure is shown.

As is seen, this type of glass demonstrates some kind of saturation. One can conclude that, after a dose of 4–6 J/cm2 , changes in the RIMA do not depend on exposure and are only affected by heat treatment. Our studies show that this kind of glass can acquire almost the same change in the refractive index as fluoride ones. The maximum value of RIMA was found to be ~11 × 10−4.

This type of glass allows for recording the mixed amplitude-phase gratings alone, and it is hard to find an application for such gratings. But since it was shown that AIMA is much smaller than it was expected to be, and on the other hand, RIMA is as strong as in fluoride glass, one can make quite a promising suggestion that bleaching of this glass would not affect the RIMA component of the grating. Therefore, we can utilize positive refraction index change with its rather big value of 1 × 10−3.

### **5.3. Bromide PTR glass**

Gratings recoded on this glass are colored (**Figure 17**), which is why it is natural to assume the modulation of absorption in the grating. The first measurements performed on these gratings proved the validity of this assumption. As one can see in **Figure 18**, the angular response from the grating, indeed, has a poor symmetry. So far measurements were only at a single wavelength of 632.8 nm that is far enough from the resonance band of silver nanoparticles. Hence, the value of the absorption index modulation amplitude (AIMA) was expected to be rather

**Figure 18** shows the approximation of experimentally obtained angular response in zero order with a theoretical curve. The position of the central maximum of the latter is shifted, and the positions of side lobes are perfectly fitted to the experimental curve. Differences in the intensities of the side lobes are connected with scattering in the sample that is inflicted by silver nanoparticles. It is also clear that this grating has a strong RIMA because the shape of the

On the other hand, our theoretical analysis shows that the AIMA is quite weak compared with other materials. The reason can be due to the fact that the measurements are conducted in a region lying far enough from the main resonance band of the silver nanoparticles. For instance,

to be almost 6 cm−1, which consists 85% of the total value of the absorption coefficient at this wavelength (7.12 cm−1). The fact that AIMA is a bit less than the latter can be explained in two ways. First, the occurrence of scattering during the recording process might create clusters outside the interference pattern, hence, lowering the contrast. Second, as seen in figure below, glass is colored even outside the irradiated region (pale red color); this can be also the reason for an additional increase in the absorption coefficient that is not connected with AIMA.

is found

AIMA for a sample subjected to the thermal treatment for 30 h and exposure of 4 J/cm2

contour includes a lot of side lobes and actually lacks the central maximum.

**Figure 19.** Typical dependence of the RIMA on exposure for chloride PTR glass.

small.

450 Holographic Materials and Optical Systems

As mentioned above, bromine PTR glass is characterized by mechanism of induced refractive index variation nearly in the same manner as that for chloride glasses. Now, bromine PTR glass remains to be a novel material that is not well investigated yet. Data available by now indicate this kind of glass to have very low refractive index change compared with that, for example, of chloride PTR glasses. A preliminary investigation of holographic properties of this material shows that, if we record a grating on the latter, the RIMA magnitudes are quite low and do not exceed 1 × 10−4. Like the spectra of chloride glasses, those of bromide ones have the absorption band in the visible region, which means that holograms recorded on this glass are mixed, i.e., have both RIMA and AIMA. Up to now, it remains unclear what is the reason for such a low contrast of the refractive index in the gratings, although the absolute refractive index variation was shown to be at least as high as 8 × 10−4. There can be several possible reasons responsible for this effect. One of the reasons can be the high scattering in the material during the recording process that leads to a decrease in the contrast of the interference pattern inside the medium. Second possible reason can be the mobility of the clusters inside the medium during the process of heat treatment. We have already demonstrated this effect for chloride PTR glasses in which the nonirradiated areas had slightly colored regions around the gratings. This area has a color the same as that of the gratings themselves, which is why we can assume that silver nanoparticles have a shell structure the same as that inside the irradiated area; hence, a decrease in the contrast. It is unclear, however, why these effects are weaker in chloride glass, thus allowing for gaining the much greater contrast of the refractive index compared with that attainable with bromide glasses.

A typical dependence of the RIMA for bromide PTR glass is shown in **Figure 20**. As seen, the dependence reveals some kind of saturation and reaches its maximum around 4 J/cm2 with no further changes. This pattern is similar to that observed for chloride glass and differs from that observed for fluoride glass. This can be taken as a proof that the mechanisms responsible for the refractive index modulation in the bulk of these two glass types are similar. In summary, we can state that, so far, the RIMA in the gratings on the bromide PTR glass is rather low and does not exceed 1 × 10−4, though this value differs from that obtained with absolute measurements, ~8 × 10−4.

**Figure 20.** Typical dependence of the RIMA on exposure for bromide PTR glass.
