Gayvoronsky V. Nonlinear optical properties study of composites based on porous semiconductors and nanostructured dielectrics.

The thesis contains study results of the nonlinear optical (NLO) response of porous semiconductors based on oxides and dielectric single crystalline and liquid crystal matrices with incorporated nanostructures of the semiconductors and organic dye molecular aggregates correspondingly. It was shown that resonance excitation of the nanoscale guest systems - interface states dealing with oxygen vacancies at the developed surface of the nanocomposites or dye molecules in aggregate - caused the sensitive photoinduced variations of the refractive index and optical absorption. The variations can be utilized both for the nonlinear optics response of the whole composite system, and for the diagnostics of comprehensive functional materials properties. For the first time the giant nonlinear optical response in porous layers of the TiO2 anatase modification nanocrystals was observed. The effect efficiency is an about six orders of magnitudes higher than the bulk material response. Due to the manifestation of the phenomenon the essential gain of the frequency conversion efficiency was obtained for the KDP:TiO2 single crystal with incorporated TiO2 nanocrystals versus the pure KDP matrix. It was shown that the cubic hyperpolarizability of the system based on nematic liquid crystal (LC) with anthraquinone dye quadratically depends on dye concentration (3)(532 nm) ~ cw2 under pulsed laser excitation, while under resonant CW excitation of the dye molecules at 633 nm the dependence is cubic. The last was explained with the cooperative effect of the dye subsystem in LC matrix. Sign inversion effect of the optical torque on the nematic director enhanced by anthraquinone dye dopants stable to the light action was revealed. The data suggest a presence of two different kinds of dipoles in the system: those absorbing light and those giving rise to the nonlinear refractive index. It was suggested that angle ~ 50o corresponds to the molecular aggregates axis direction towards the n0 in LC matrix.