**3. Properties of chloride PTR glasses**

mechanical and optical strength and also reveal, from this point of view, practically no difference with the commercial optical glass BK7 (Schott). 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). The important advantages of PTR glass as the optical

**i.** High optical homogeneity (the refraction index fluctuations across the glass bulk are

**ii.** Reproducibility of its parameters from one glass synthesis to another and also in the

**iii.** PTR glass can be subjected, similar to optical glass BK7, to various kinds of both the

**iv.** One can fabricate PTR glass both in the laboratory conditions (hundreds of grams)

gies such as molding, aspheric surface production, and drawing fiber.

Mechanical and thermal resistance, chemical durability Close to commercial optical glass BK7

mechanical processing such as grinding and polishing and the formation technolo-

and industrial ones (hundreds of kilograms) using a simple and nontoxic technology. The chemical reagents required for the glass fabrication are commercially available

(Data of University of Central Florida)

(Data of University of Central Florida)

course of the photo-thermo-induced crystallization.

Transparency range, nm 350–3000 Photosensitivity spectral range, nm 280–350 Photosensitivity, mJ/cm2 50 RI change, Δn, ppm 1000 RI modulation amplitude, δn, ppm 500

**•** visible range 0.1 **•** near IR range 0.01

**•** CW regime, kW/cm2 10 **•** Nanosecond pulse regime, J/cm2 40

Diffraction efficiency, % 95 Hologram thickness, mm 0.1–10 Angular selectivity, ang. min <1 Bandwidth FWHM, nm 0.1

VBGs are completely stable at temperature, °C 500 VBGs are completely stable at temperature, °C 500

**Table 1.** Characteristics of PTR glass and VBGs recorded on the glass.

Spatial frequency VBGs, mm−1 up to 10,000

Size, mm up to 25 × 25

medium are as follows:

440 Holographic Materials and Optical Systems

Induced optical loss, cm−1

of the order of 10−5).

and not too expensive.

Optical resistance (laser-induced damage threshold):

The chloride PTR glasses are photosensitive multicomponent glasses based on Na2O-ZnO-Al2O3-SiO2-NaF system doped with variable batch concentration of Cl (0–2.2 mol%), a photosensitizer such as CeO2 (0.01 mol%), a reducer such as Sb2O3 (0.05 mol%), and also Ag2O (0.15 mol%). The chloride PTR glass was designed and synthesized in ITMO University, Russia [24].

**Figure 4.** The evolution of the absorption spectra of PTR glass containing 2.2 mol% Cl during photo-thermo induced cristallization (1) is the spectrum for initial untreated glass, (2) is that for glass after the UV irradiation for 50 s alone, (3) is the spectrum for glass after the heat treatment alone at 550°C for 3 h, and (4) is the spectrum for glass after the UV irradiation for 50 s and subsequent heat treatment at 550 °C for 3 h. An inset shows the photos and absorption spectra (700–2500 nm) of treated chloride PTR glass samples containing 2.2 mol% Cl. Here (1) is initial untreated glass, (2) is glass after the UV irradiation for 50 s alone, (3) is glass after the heat treatment alone, and (4) is glass after the UV irradiation for 50 s and subsequent heat treatment.

With changing the type of halide (fluoride to bromide or chloride) in the PTR glass composition, it is possible to control the sign of the RI increment. As mentioned above, for the case of fluoride PTR glass, thermal treatment at temperatures higher than *Tg* results in a decrease in the RI of the UV-irradiated area in comparison with that of nonirradiated area. On the other hand, the substitution of fluorine by chlorine leads to the precipitation of nano-crystalline phases of mixed silver and sodium chlorides in glass host and to the positive increment of RI (∆*n* up to 1.0 × 10−3) [24].

**Figure 5.** Scheme for the photo-thermo-induced crystallization mechanism inherent in chloride PTR glasses for various Cl concentrations (0–2.2 mol%). (a) Is the growth of shell-free silver nanoparticles in glasses containing 0–1.0 mol% Cl. (b) Is the growth silver nanoparticles with a shell composed of mixed silver and sodium chlorides in glasses containing 0–2.2 mol% Cl.

**Figure 6.** Photos of chloride PTR glass luminesce under UV (λ = 365 nm) excitation (a) is the photo of PTR glass containing 1 mol% Cl after the UV irradiation with various doses. The exposure duration (sec) that sets a dose is (1) 0.5 s, (2) 1 s, (3) 5 s, (4) 50s, (5) 500 s; (b) is the photo of the UV-irradiated and heat treated (1 h 400°C) chloride PTR glasses differing in the chlorine concentration. The chlorine concentrations (mol%) being (1) 0, (2) 1, and (3) 2.

Initially, chloride PTR glasses are transparent in a wide spectral range. 250–2500 nm (**Figure 4**). The UV irradiation of chloride PTR glasses results in the Ce3+ ion photoionization and the resultant formation of silver molecular clusters (SMC), the latter playing the role of crystallization centers (**Figures 4** and **5**). Heating all studied chloride PTR glasses at temperatures above 250°C and less than *Tg* results, as shown in **Figure 5(b)**, in releasing electrons from Sb and capturing them by Ag ions with further formation of an extra amount of neutral silver atoms and molecular clusters [19]. The latter provide, according to Refs. [27, 28], a broadband luminescence in the visible and NIR ranges (**Figure 6**). Further, the heat treatment of PTR glasses containing 0–1.0 mol% Cl at temperatures above *Tg* leads to the precipitation of silver nanoparticles with no shell (**Figure 5**). At the same time, such treatment of PTR glasses containing >1.0– 2.2 mol% Cl results in the precipitation of silver nanoparticles with a shell consisting, according to Ref. [24], of mixed sodium and silver chlorides in a varied proportion (**Figures 4** and **5**). The evolution of absorption spectra during the photo-thermo-induced crystallization is shown in **Figure 4**. It can be seen that the heat treatment of nonirradiated chloride PTR glass has no measurable effect on the absorption spectra. According to calculations described in Ref. [24], the sizes of silver nanoparticles and silver and sodium chloride nanocrystals are relatively small (about 3 nm [**Figure 7**] for silver nanoparticles, NPs, and 27 nm for nanocrystals). This is why chloride PTR glasses exhibit a rather low level of scattering.

**Figure 7.** TEM image of silver nanoparticles in chloride PTR glass. Scale − 50 nm.

hand, the substitution of fluorine by chlorine leads to the precipitation of nano-crystalline phases of mixed silver and sodium chlorides in glass host and to the positive increment of RI

**Figure 5.** Scheme for the photo-thermo-induced crystallization mechanism inherent in chloride PTR glasses for various Cl concentrations (0–2.2 mol%). (a) Is the growth of shell-free silver nanoparticles in glasses containing 0–1.0 mol% Cl. (b) Is the growth silver nanoparticles with a shell composed of mixed silver and sodium chlorides in glasses containing

**Figure 6.** Photos of chloride PTR glass luminesce under UV (λ = 365 nm) excitation (a) is the photo of PTR glass containing 1 mol% Cl after the UV irradiation with various doses. The exposure duration (sec) that sets a dose is (1) 0.5 s, (2) 1 s, (3) 5 s, (4) 50s, (5) 500 s; (b) is the photo of the UV-irradiated and heat treated (1 h 400°C) chloride PTR glasses

Initially, chloride PTR glasses are transparent in a wide spectral range. 250–2500 nm (**Figure 4**). The UV irradiation of chloride PTR glasses results in the Ce3+ ion photoionization and the resultant formation of silver molecular clusters (SMC), the latter playing the role of crystallization centers (**Figures 4** and **5**). Heating all studied chloride PTR glasses at temperatures above 250°C and less than *Tg* results, as shown in **Figure 5(b)**, in releasing electrons from Sb and capturing them by Ag ions with further formation of an extra amount of neutral silver atoms and molecular clusters [19]. The latter provide, according to Refs. [27, 28], a broadband luminescence in the visible and NIR ranges (**Figure 6**). Further, the heat treatment of PTR glasses containing 0–1.0 mol% Cl at temperatures above *Tg* leads to the precipitation of silver nanoparticles with no shell (**Figure 5**). At the same time, such treatment of PTR glasses containing >1.0– 2.2 mol% Cl results in the precipitation of silver nanoparticles with a shell consisting, according to Ref. [24], of mixed sodium and silver chlorides in a varied proportion (**Figures 4** and **5**). The evolution of absorption spectra during the photo-thermo-induced crystallization is shown in **Figure 4**. It can be seen that the heat treatment of nonirradiated chloride PTR glass has no measurable effect on the absorption spectra. According to calculations described in Ref.

differing in the chlorine concentration. The chlorine concentrations (mol%) being (1) 0, (2) 1, and (3) 2.

(∆*n* up to 1.0 × 10−3) [24].

442 Holographic Materials and Optical Systems

0–2.2 mol% Cl.

**Figure 8.** Effect of chlorine 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.

**Figure 8** shows the evolution of the refractive index of PTR glass with an increase in the chlorine concentration for initial, heat-treated, and UV-irradiated and then heat-treated glasses (curves 1–3). As seen, the incorporation of Cl results in a consecutive increase in the refractive index of glass irrespective of treatment applied. In particular, Curves 1 and 2 coincide with each other, i.e., the heat treatment of nonirradiated chloride PTR glasses does not change their refractive index [24]. On the contrary, the UV irradiation and subsequent heat treatment of chloride PTR glasses result in a significant increase in their refractive index. For the maximum chlorine concentration, a difference Δ*n* between the refractive index values of the UV-irradiated and nonirradiated glasses after the heat treatment reaches magnitudes up to 1.0 × 10−3. The positive increment of refractive index as well as high value of Δ*n* in chloride PTR glasses can be used for recording VBGs and optical waveguides.

### **4. Properties of bromide PTR glasses**

The bromide PTR glasses are photosensitive multicomponent glasses based on Na2O-ZnO-Al2O3-SiO2-NaF system doped with variable batch concentration of Br (0–1.5 mol%), photosensitizer, such as CeO2 (0.01 mol%), reductant, such as Sb2O3 (0.05 mol%), and Ag2O (0.1 mol %). The bromide PTR glass was designed and synthesized in ITMO University, Russia [17].

**Figure 9.** Absorption spectra of PTR glass containing 0.7 mol% Br (1) is the spectrum of initial untreated glass, (2) is the spectrum for glass after the UV irradiation for 50 s alone, (3) is the spectrum for glass after the heat treatment alone, and (4) is the spectrum for glass after the UV irradiation for 50 s and subsequent heat treatment. An inset shows the photos and absorption spectra (700–2500 nm) of treated bromide PTR glass samples containing 0.7 mol% Br. Here (1) is initial untreated glass, (2) is glass after the UV irradiation for 50 s alone, (3) is glass after the heat treatment alone, and (4) is glass after the UV irradiation for 50 s and subsequent heat treatment.

Initially, bromide PTR glasses are transparent in a wide range: 250–2500 nm (**Figure 9**). The substitution of chlorine by bromine in PTR glass composition affects the crystallization mechanism (**Figure 10**). The UV irradiation of bromide PTR glasses results in the Ce3+ ion photoionization and SMC formation (**Figure 9**); the latter playing the role of crystallization centers (**Figure 3(a)**). Heating all the studied bromide PTR glasses at temperatures above 250 C and less than *Tg* results, as shown in **Figure 3(b)**, in releasing electrons from Sb and capturing them by Ag ions with further formation of an extra amount of neutral silver atoms and molecular clusters [19]. The latter provides, according to Refs. [27, 29], a broadband luminescence in the visible and NIR ranges. Further, the heat treatment of PTR glasses containing 0.25– 0.7 mol% Br at temperatures above *Tg* leads to the precipitation of silver nanoparticles with a silver bromide-based shell varying in thickness [20] and/or composition [30]; namely, mixed silver and sodium bromides can occur (**Figure 10(a)**). Moreover, the above heat treatment can result in shifting the plasmon resonance absorption band toward the greater wavelengths (**Figure 9**); such a treatment of PTR glasses containing from 1.0 to 1.5 mol% Br results in the precipitation of small silver nanonoparticles without a perceptible plasmon resonance peak in the absorption spectrum (**Figure 11**)—the nanoparticles being covered by a shell consisting of silver bromide (**Figure 10(b)**). The sizes of silver nanoparticles and silver bromide nanocrystals are relatively small (<3 nm for silver NPs and <11 nm for nanocrystals).

positive increment of refractive index as well as high value of Δ*n* in chloride PTR glasses can

The bromide PTR glasses are photosensitive multicomponent glasses based on Na2O-ZnO-Al2O3-SiO2-NaF system doped with variable batch concentration of Br (0–1.5 mol%), photosensitizer, such as CeO2 (0.01 mol%), reductant, such as Sb2O3 (0.05 mol%), and Ag2O (0.1 mol %). The bromide PTR glass was designed and synthesized in ITMO University, Russia [17].

**Figure 9.** Absorption spectra of PTR glass containing 0.7 mol% Br (1) is the spectrum of initial untreated glass, (2) is the spectrum for glass after the UV irradiation for 50 s alone, (3) is the spectrum for glass after the heat treatment alone, and (4) is the spectrum for glass after the UV irradiation for 50 s and subsequent heat treatment. An inset shows the photos and absorption spectra (700–2500 nm) of treated bromide PTR glass samples containing 0.7 mol% Br. Here (1) is initial untreated glass, (2) is glass after the UV irradiation for 50 s alone, (3) is glass after the heat treatment alone, and

Initially, bromide PTR glasses are transparent in a wide range: 250–2500 nm (**Figure 9**). The substitution of chlorine by bromine in PTR glass composition affects the crystallization mechanism (**Figure 10**). The UV irradiation of bromide PTR glasses results in the Ce3+ ion photoionization and SMC formation (**Figure 9**); the latter playing the role of crystallization centers (**Figure 3(a)**). Heating all the studied bromide PTR glasses at temperatures above 250 C and less than *Tg* results, as shown in **Figure 3(b)**, in releasing electrons from Sb and capturing them by Ag ions with further formation of an extra amount of neutral silver atoms and molecular clusters [19]. The latter provides, according to Refs. [27, 29], a broadband luminescence in the visible and NIR ranges. Further, the heat treatment of PTR glasses containing 0.25– 0.7 mol% Br at temperatures above *Tg* leads to the precipitation of silver nanoparticles with a silver bromide-based shell varying in thickness [20] and/or composition [30]; namely, mixed silver and sodium bromides can occur (**Figure 10(a)**). Moreover, the above heat treatment can result in shifting the plasmon resonance absorption band toward the greater wavelengths (**Figure 9**); such a treatment of PTR glasses containing from 1.0 to 1.5 mol% Br results in the

be used for recording VBGs and optical waveguides.

(4) is glass after the UV irradiation for 50 s and subsequent heat treatment.

**4. Properties of bromide PTR glasses**

444 Holographic Materials and Optical Systems

**Figure 10.** Scheme for the photo-thermo-induced crystallization mechanism inherent in bromide PTR glasses for various Br concentrations (0–1.5 mol%). (a) The photoactivation of PTR glass (Ce3+ ion photoionization), formation of neutral silver molecular clusters, and capturing electrons by Sb5+ valence states. (b) Releasing electrons by Sb and capturing them by Ag ions with the formation of neutral silver atoms and clusters, and (c) The growth of (i) silver nanoparticles with a shell composed of silver bromide in glasses containing 0.25–0.7 mol% Br or (ii) small silver nanoparticles characterized by broad plasmon resonance peak with a shell made of silver bromide nonocrystals in glasses containing 1–1.5 mol% Br.

**Figure 11.** Absorption spectra of PTR glass containing 1 mol% Br (1) is the spectrum for initial untreated glass, (2) is the spectrum for glass after the heat treatment alone, (3) is that for glass after the UV irradiation for 50 s alone, and (4) is the one for glass after the UV irradiation for 50 s and subsequent heat treatment. An inset shows the effect of bromine concentration on the average size of silver nanoparticles (NP) calculated using Mie theory and the photos of bromide PTR glass containing 1 mol% Br at all stages of photo-thermo-induced crystallization (1) initial untreated glass, (2) glass after the heat treatment alone, (3) glass after the UV irradiation for 50 s alone and (4) is the one after the UV irradiation for 50 s and subsequent heat treatment.

**Figure 12** shows the evolution of the PTR glass refractive index with an increase in the bromine concentration for initial, heat-treated, and UV-irradiated and then heat-treated glasses (curves 1–3). As is shown, the incorporation of Br leads to a consecutive increase in the refractive index of glass irrespective of treatment applied. In particular, curves 1 and 2 coincide with each other up to reaching the bromine concentration of 0.7 mol%. In other words, the heat treatment of 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 nonirradiated glasses after the heat treatment reaches magnitudes up to 0.8 × 10−3.

**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.
