**2. Properties of fluoride PTR glasses**

Corning, Inc., in 1977 and were referred to as polychromatic (PC) glasses [1–3]. In Russia, similar photosensitive glasses [4, 5] were denoted by multichromatic (MC) ones. PC/MC glasses are known to contain, in addition to Na2O, Al2O3, ZnO, and SiO2, some other ingredients such as (i) photosensitive dopants playing the roles of electron donors (Ce3+) and

formation of the crystalline phases. The main specific feature of PC/MC glasses is the selective absorption in the visible. Namely, PC glasses can acquire, under the effects of the UV exposure and subsequent heat treatment, a wide variety of colors. In brief, the final stages of photochemical and diffusion processes responsible for this coloration were assumed to be as follows. The colloidal silver particles formed under the above effects play the role of nucleation centers. Around such centers, the growth of NaF and (Ag, Na)Br nano- or microcrystals occurs. Under particular growth conditions, the microcrystals acquire complicated anisotropic shapes such as the elongated pyramid-like structures stretched along an axis [1, 3, 6]. Additional multistage UV irradiation and heat treatment lead to the photolytic precipitation of silver layer on the surfaces of these anisotropic structures (so-called "decoration of the latter with silver"). The anisotropy of metallic silver shells thus formed results in a certain shift of the corresponding absorption band into the visible. So the substantial anisotropy of metallic silver particles was considered to be the principal condition for the

In the late 1980s–early 1990s [7–10], it was proposed first in Vavilov State Optical Institute to apply PC/MC glasses for recording the 3D phase holograms. Unlike the case of PC/MC coloration, only a single stage of photo-thermo-induced crystallization was used, and this stage included the UV irradiation and subsequent heat treatment. When developing the corresponding procedures, the principal attention was paid to a difference obtainable in the refractive indices of vitreous and crystalline phases rather than the anisotropic shapes of microcrystals. As a result, a new class of materials was developed in Vavilov State Optical Institute, this class was denoted [10] by a specific term such as "photo-thermo-refractive (PTR) glasses" (i.e., glasses whose refractive index varies due to the UV irradiation and subsequent heat treatment). Later [11, 12], this term started to be used widely in other countries as well. Now, there is an increased interest in PTR glasses because the volume Bragg gratings recorded on these glasses reveal a unique combination of working characteristics such as the high angle and spectral selectivity, high diffraction efficiency, high mechanical and optical strength, and also high thermal and chemical durability. Based on PTR glasses, a broad variety of optical devices are developed including extra narrow-band filters, wavelength division multiplexing (WDM) devices, combiners of high-intensity light beams, chirped gratings for compressing

the light impulses, filters for increasing the spectral brightness of laser diodes, etc.

The given paper is a survey of recent achievements of ITMO University (St. Petersburg, Russia) in developing new holographic media such as fluoride, chloride, and bromide silicate photothermo-refractive (PTR) glasses as well as the holographic diffractive optical elements that are the volume Bragg gratings recorded in the glasses for improving dramatically the parameters

) that participate in the

, Sb5+, and Sn4+) and also (ii) halogen ions (F− and Br−

occurrence of "PC/MC coloration" in PC/MC glasses.

of laser systems of different types.

acceptors (Ag+

436 Holographic Materials and Optical Systems

The fluoride PTR glass was designed and synthesized in ITMO University, Russia [12]. The fluoride PTR glass is a photosensitive multicomponent sodium-zinc-alumino-silicate one containing fluorine (6 mol%) and small amount of bromine (0.5 mol%) and also doped with additives (cerium, antimony, and silver) that are responsible for the photo-thermo-induced precipitation of silver nanoparticles and sodium fluoride crystals—see for example [1, 13, 14]. Untreated fluoride PTR glasses are transparent in a wide spectral range of 250–2500 nm (**Figure 1(a)**). The selective UV irradiation into the Ce3+ absorption band in the spectra of these glasses results in the formation of neutral silver molecular clusters. The subsequent heat treatment of UV-irradiated PTR glasses near the glass transition temperature (*Tg*) induces the silver nanoparticle formation [1] (**Figure 1(b)**). The thermal treatment of these glasses at temperatures above *Tg* leads to the growth of silver bromide shell on a silver nanoparticle [15] and then to the precipitation of sodium fluoride cone on it [1, 16]. Image of XRD pattern of UVexposed and thermal-treated PTR glass sample is shown in **Figure 2**.

**Figure 1.** Absorption spectra of (a) virgin PTR glass and (b) the glass sample after the UV irradiation and subsequent heat treatment.

**Figure 2.** X-ray diffraction pattern of UV-irradiated and thermal-treated PTR glass.

In Ref. [13], authors showed, for the first time, the dramatic effect of bromine on the process of NaF crystal growth in fluoride PTR glasses. The paper has demonstrated that the growth of sodium fluoride crystals is possible only in the presence of bromide additives in the PTR glass composition. A generalized NaF crystallization mechanism that consists of three stages is proposed in Refs. [1, 17].

The process of photo-thermo-induced crystallization of fluoride PTR glass is shown schematically in **Figure 3**.

**Figure 3.** Photo-thermo-induced crystallization of fluoride PTR glass. (a) Cerium photoionization and trapping the photoelectrons by Sb; (b) Releasing electrons Sb and trapping them by Ag ions with the formation of neutral silver atoms and clusters; (c) Formation of colloidal particles under heating up to 400°C; (d) Growth of (Ag, Na)Br shell on colloidal silver particles at *T* > 500°C; (e) Growth of NaF microcrystals at *T* > 500°C.

At the first stage, the trivalent cerium ion donates an electron under the effect of the UV irradiation, thus increasing its own valency in accordance with the following reaction (**Figure 3(a)**)

$$\text{Ce}^{3+} + \text{h}\,\nu \to \text{e}^{-} + \left[\text{Ce}^{3+}\right]^{+} \tag{1}$$

Released photoelectrons can be trapped partially by silver ions (~20%) with subsequent neutral silver atom and molecular cluster formation (Ag0 , Ag2 0 , Ag2 + , Ag3 2+), but most photoelectrons are trapped by antimony ions according to the following reaction (**Figure 3(b)**):

New Photo-Thermo-Refractive Glasses for Holographic Optical Elements: Properties and Applications http://dx.doi.org/10.5772/66116 439

$$\text{Fe}^{\cdot} + \text{Sb}^{\text{s}\cdot} \rightarrow \left[\text{Sb}^{\text{s}\cdot}\right]^{\cdot} \tag{2}$$

At the second stage, the heat treatment at relatively low temperatures (300–450°C) leads to releasing the trapped electrons from antimony (**Figure 3(c)**) with further formation of silver molecular clusters and colloidal nanoparticles (**Figure 2(b)**):

In Ref. [13], authors showed, for the first time, the dramatic effect of bromine on the process of NaF crystal growth in fluoride PTR glasses. The paper has demonstrated that the growth of sodium fluoride crystals is possible only in the presence of bromide additives in the PTR glass composition. A generalized NaF crystallization mechanism that consists of three stages is

The process of photo-thermo-induced crystallization of fluoride PTR glass is shown schemat-

**Figure 3.** Photo-thermo-induced crystallization of fluoride PTR glass. (a) Cerium photoionization and trapping the photoelectrons by Sb; (b) Releasing electrons Sb and trapping them by Ag ions with the formation of neutral silver atoms and clusters; (c) Formation of colloidal particles under heating up to 400°C; (d) Growth of (Ag, Na)Br shell on

At the first stage, the trivalent cerium ion donates an electron under the effect of the UV irradiation, thus increasing its own valency in accordance with the following reaction

Released photoelectrons can be trapped partially by silver ions (~20%) with subsequent neutral

, Ag2 0 , Ag2 + , Ag3

ë û (1)

2+), but most photoelectrons

3 3 Ce h e Ce n<sup>+</sup> + -+ + ®+ é ù

are trapped by antimony ions according to the following reaction (**Figure 3(b)**):

colloidal silver particles at *T* > 500°C; (e) Growth of NaF microcrystals at *T* > 500°C.

silver atom and molecular cluster formation (Ag0

proposed in Refs. [1, 17].

438 Holographic Materials and Optical Systems

ically in **Figure 3**.

(**Figure 3(a)**)

$$
\left[\mathbf{S}\mathbf{b}^{\mathfrak{s}^{\mathfrak{s}}}\right]^{-} \xrightarrow{} \mathbf{S}\mathbf{b}^{\mathfrak{s}^{\mathfrak{s}}} + \mathbf{e}^{-}\tag{3}
$$

$$\text{nAg}^+ + \text{ne}^- \rightarrow \text{nAg}^0 \tag{4}$$

At the third stage, the heat treatment at temperatures above *Tg* results, first, in the growth of mixed silver bromide-sodium bromide shell on a silver nanoparticle (**Figure 3(e)**) and, further, in the coaxial growth of sodium fluoride crystalline phase on this shell (**Figure 3(g)**).

In Ref. [15], authors showed that the UV irradiation and subsequent heat treatment of fluoride PTR glass induces the refractive index change only in the UV-irradiated area. There is still some uncertainty in the origin of refractive index change in PTR glass, and several presumable mechanisms of the change are discussed. Classically, this effect is assumed to be caused by difference in the refractive indices between the NaF crystal phase (*n* ~ 1.33) sedimented in the UV-irradiated area and the unexposed glass area (*n* ~ 1.49) so that the precipitation of sodium fluoride leads to the negative refractive index increment. Although a difference between the refractive indices of sodium fluoride and vitreous phase is rather big, the negative refractive index change in the UV-irradiated area does not exceed 1 × 10−3 [15, 18]. This can probably be due to the fact that, in addition to the NaF phase precipitation, there is also the silver bromide shell with a high refractive index value (*n* ~ 2.3) on the silver nanoparticle. As shown in many sources (see for example Refs. [14, 19, 20]), the maximum of surface plasmon resonance of silver nanoparticles in fluoride PTR glasses shifts to the greater wavelengths owing to the silver bromide shell growth.

On the other hand, the authors of [21] proposed another possible mechanism of photo-thermoinduced refractive index change in PTR glass. They assumed that the transformation of Na+ and F− distributed in the PTR glass matrix into the crystalline NaF (a chemical change) and structural relaxation process are not the main causes of photo-thermo-induced refractive index change and assigned this change to high residual stresses around the NaF crystals. According to calculations presented in the paper, these stresses are the most important cause for the photothermo-induced refractive index change in PTR glass.

Also, fluoride PTR glasses have outstanding mechanical, optical, and chemical properties. In particular, they show a high photosensitivity, high thermal stability of the recorded phase holograms, and high tolerance to the optical and ionizing irradiation. The basic optical and spectral properties of PTR glass are described in Refs. [14, 22–24]. The holographic optical elements (HOE)s recorded on the PTR glass demonstrate high chemical stability, thermal, 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 medium are as follows:



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

One should also note some features of PTR glass that are unusual for other recording media. For example, PTR glass can be processed with the ion exchange technology, which provides possibilities (i) to fabricate the ion-exchanged optical [25] or plasmonic waveguides and (ii) to implement the surface strengthening, thus improving the mechanical strength, chemical stability, and thermal and also optical strength.

Some characteristics and advantages of fluoride PTR glasses and also of holographic volume Bragg gratings (VBG) recorded on the glasses are presented in **Table 1** [26].
