**1. Introduction**

#### **1.1 Why glassceramics?**

Glass-crystalline materials (or glassceramics) are heterophase composite materials, usually consisting of a glassy matrix (glass phase) and micro- or nano-sized dielectric or semiconductor crystals (crystalline phase), or metallic particles distributed in it. Glassceramics are formed by the growth of crystalline phase inside of a glass matrix. The crystal growth can occur as a result of spontaneous thermal crystallization of glass, as in case of heat treatment. For example, there are following typical representatives of the spontaneous crystallization: glasses doped with microcrystals of CdS, CdSe, CdTe, PbS, PbSe (non-linear media) [1-7], glasses doped with semiconductor microcrystals AgBr, AgCl, CuBr, CuCl (photochromic media) [8], glasses doped with dielectric microcrystals Li2O-SiO2 (photoetchable media – FOTURAN TM, FOTOFORM TM, PEG TM [9-11]. The crystal growth can occur as a result of photo-thermo-induced crystallization caused by UV photoirradiation and subsequent heat treatment. In this case, UV radiation generates centers of nucleation and the thermal treatment results in the growth of microcrystals in irradiated area of the glass host. Glasses doped with complicated microcrystals of NaF-AgBr (polychromatic glasses [12] and photo-thermo-refractive - PTR glasses [13]) are typical representatives of the photo-thermo-induced crystallization.

#### **1.2 Why nanostructured glassceramics?**

One of the major drawbacks of glass ceramic materials is a high light scattering occurring at the boundary of crystalline phase and glass phase. That is why current research in the development of optical glass-crystalline materials is aimed at decreasing of light scattering by means of formation of nanosize (5-30 nm) crystals or nanoparticles in the glass matrix. Only the nanoscale nature of the crystalline phase can significantly reduce the light scattering in heterophase composites (where the extinction coefficient can reach less than 0.01 cm-1) and classify these materials as optical. Fig.1 illustrates transition from millimetersize crystals to micrometer-size crystals and finally nano-size crystals in the glass host. The transition to the nanoscale crystalline phase not only leads to changes in physical, chemical and optical properties of glassceramics, but is also a cause for fundamentally new and

New Nanoglassceramics Doped with Rare Earth Ions and Their Photonic Applications 107

attention do to the series of spectroscopy advantages. It is obvious that from the point of view of laser active media development optimal are the materials, which are characterized by the low-frequency phonon spectrum and by the low content of the OH-groups, because in this case one can reduce the excitation energy losses due to the multi-phonon quenching process. For a long time there was a common opinion that only the fluorine-containing materials (like fluoride glasses and crystals) are optimal for the said problem solution. However, since recently the synthesis of the glassceramics materials like the oxifluoride silicate glasses has become the priority direction of studies [16-24]. Such composite materials combine the optical parameters of the low-phonon fluoride crystals and the good mechanical and chemical features of the silicate glasses. It was also revealed that some of the oxifluoride glass-like materials have a feature of forming the fluoride nanocrystals, doped by the rare-earth ions, during the process of heat treatment of the raw primary glass. Hence such a materials combine all the positive features of the fluoride nanocrystals, which control the optical properties of the rare earth ions, with that of oxide glasses like easy production technology and excellent macroscopy features like chemical and mechanical strength and the high optical quality. It is well known that in the case of production of the optically transparent glassceramics, including those, which are used for optical waveguide fabrication, it is very important to minimize the light losses for absorption and scattering. The Rayleigh scattering by the micro-inhomogeneities with the size about the radiation wavelength is a factor, limiting such materials use. It imposes the strict limitations over the size of the separated crystalline phase. According to the Rayleigh theory for the visible spectral range, the radius of the crystals, dispersed around the glass, has to be not more than 15 nm. The refraction indexes of the crystalline phase and of the amorphous matrix are to differ in not more than 0.1. Later on these limitations were somewhat softened. In [16] on the case of a model it was shown that one can produce the transparent glassceramics with the size of nanocrystals up to 30 nm and with the refraction indexes

The separation of the crystalline phase in a silicate glass is a traditional way of the opalescent glass production. It is well known that insertion of the fluorides into the glass, whose content is similar to the window glass, leads to forming of a large number of microcrystals in the glass volume. It leads to a drastic increase of a light scattering and to the so called opalescence effect. Hence, the fluorine content in a glass leads to intense phase separation; very often the second phase is represented by the introduced fluorides or by their derivatives. This fact has became the basis for the numerous studies, devoted to fabrication and investigation of the silicate nanoglassceramics, based on the fluorite-like nanocrystals like Ba(Sr,Ca,)F2 or of the hexagonal LaF3, activated by small concentration or

The separate group is represented by the glassceramics, fabricated on the basis of the glasslike systems with a big amount of fluorides. One can fill into this type the alkali-less germanate and silicate systems like GeO2-PbO-PbF2, SiO2-PbO-10PbF2 SiO2 -Al2О3- CdF2 - PbF2- ZnF2:(YF3). In such systems the fluoride concentration can be as high as 60-70 mol.%. The process of crystalline phase formation in this case is not so obvious and needs a more

difference in not more than 0.3.

rare-earth or of transient elements.

detailed study.

**2.1 The glassy matrix for glassceramics production** 

unique properties. For example, nano-size of crystalline phase results in quantum-size, resonance, and other effects. Such materials can possess unique properties, that can't be realized in traditional materials. Following new materials can be synthesized on the base of nanoglassceramics: plasmonic materials, photonic crystals, metha-materials.

Currently, optical transparent glassceramics are of the great interest for the modern element base of photonics, because they stay at intermediate state between crystalline materials and glasses. These glassceramics combine the best properties of crystals (high emission crosssection, quantum yield of luminescence, mechanical and thermal strength etc.) and glasses (possibilities of pressing and molding, spattering, pulling optical fibers, and carrying out ion exchange to fabricate waveguide structures).

In this paper a new type of glassceramics, namely the nanostructured lead oxifluoride glasseramics, which was developed by the authors have been discussed. In addition some examples of using the novel nanoglassceramics for photonics applications have been outlined.

Fig. 1. Transition from millimeter-size crystals (a) to a micrometer-size crystals (b) and to nano-size crystals (c) in the glass host.
