Luminescence and nonlinear optical properties of hybrid associates of Ag$_{2}$S quantum dots with molecules of thiazine dyes

Spectral, kinetic, and nonlinear optical regularities that demonstrate the exchange of electronic excitations between the components of hybrid associates of Ag$_{2}$S colloid quantum dots (1.7–1.8 nm) in gelatin with molecules of thiazine dyes (Ds) are found. When the IR luminescence of Ag$_{2}$S quantum dots (QDs) is excited by radiation from the thionine absorption region, its enhancement due to nonradiative resonant energy transfer is observed. The association with methylene-blue molecules blocked the IR luminescence of Ag$_{2}$S QDs upon its excitation by radiation from the absorption region of the dye due to the transfer of charge carriers. It is demonstrated that the hybrid association of thionine molecules and Ag$_{2}$S QDs adversely affects the nonlinear optical properties of the latter, which manifests itself in inverse saturated absorption by the action of 10-ns second-harmonic pulses (532 nm) of a Nd$^{3+}$:YAG laser. For the associates of Ag$_{2}$S QDs with methylene-blue molecules, the radiation focusing caused by the transfer of charge carriers from the dye and the change in the population of small traps in nanocrystals is found. It is concluded that the direction of the transfer of electronic excitations and the photophysical processes in these objects are determined by the mutual arrangement of the HOMO–LUMO levels of the dye with respect to the levels of dimensional quantization of the Ag$_{2}$S QDs.