Vibrational spectra and nonlinear optical phenomena of nanocrystals (TeO₂) in pores of an opal

The creation and investigation of the nano-sized active dielectric crystals attracts great attention in physics of low-dimensional systems. Physical properties of such nanocrystals should be essentially different from those of bulk crystals because of the quantum-size effects in electronic and vibrational spectra ^[1]. On the other hand, in case of periodical arrangement of the nanocrystals, effects attributed to photonic crystals can be expected also. Thus, such structures may increase our opportunities in control of the flow of light.

As a base matrix for obtaining these structures synthetic opals are widely used. Regular arrangement of cavities and channels in pristine synthetic opals allows getting 3D-periodical structures for a wide range of organic and inorganic compounds. A typical embedding procedure is a soaking of an initial opal with the compound solution at temperatures close to room temperature or into the melt of compound followed by crystallization by cooling ^{[3, 4]}. By embedding active dielectrics from the melt a higher degree of filling (up to 60 % from the pores volume) is achieved, and the surface and interior pores are filled with quite equal compound quantity ^[3]. Besides, in this case, a solid phase of embedded compound is formed.

These special conditions are the high temperature, the limited volume of pores and the lack of atmosphere into interior pores. Taking these factors into account, one should not exclude the possibility of formation of metastable polymorphs of dielectrics into pores because of the interaction between the melt and the SiO₂ globule surface.

The present work is devoted to growing tellurium dioxide into pores of synthetic opals and to study Raman spectra of obtained composites with the aim of revealing the possible structural changes and size effects in vibrational spectra. The choice of tellurium dioxide is due to its strongly pronounced piezoelectric, acousto-optic and nonlinear optical properties ^{[5, 6]}, which provide a wide range of applications, e.g. as a material for optical fibers and nonlinear optical elements ^[7]. Besides, three crystalline polymorphs of tellurium dioxide have been obtained by now ^{[8 - 11]}. One of them, α-TeO₂, synthesized from TeO₂-rich glasses with different initiating oxides (WO₃, Nb₂O₅, PbO) is metastable ^[11].

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