The features of degenerate four-wave mixing by means of two-photon excitation of exciton transitions in a colloidal solution of CdSe/ZnS quantum dots (QDs) were revealed. This process was studied by means of dynamic Bragg gratings creation in colloidal QDs solution. The intensity of the generated pulses as a result of the degenerate four-wave interaction should be proportional to the cube of the incident pulses intensity. However, the 6th degree dependence and the quadratic dependence were experimentally measured for the first half and for the second half of the pulses in the train, correspondingly. To determine the features of the two-photon absorption the dependence of the intensity of the transmitted and self-diffracted pulses on the incident pulses intensity was measured. The experimental results can indicate a simultaneous effect on the transmission and refraction of the colloidal solution of QDs in the case of resonant two-photon excitation of the basic exciton transition as a non-inertial (classical) nonlinearity and resonance nonlinearity associated with a strong exciton absorption.
The monocrystalline Bi2Te3-xSex and Bi2-xSbxTe3-ySey films of various compositions and thickness were synthesized by MOCVD on sapphire substrates1 . The transmission of laser pulses (duration τ≈35 ps, wavelength λ=1064 nm) through the films was measured as a function of radiation intensity with the aim to optimize functionality of these films deposited on different substrates as saturable absorbers for the application in passively mode-locked 1-2 μm wavelength lasers. There was investigated nonlinear absorption of films of various compositions and thickness with measured saturation intensity ≈ 20 MW/cm2 for Bi2Te3-xSex and 45 MW/cm2 for Bi2-xSbxTe3-ySey. Data obtained can be explained by the classical nonlinearity – phase space filling in a quantum well2 . As it was established earlier3 , the semiconductor conductivity type of the film without light interaction can be changed to the metallic conductivity type in the case of light interaction with the film, that is strongly nonlinear depends on the light intensity. At the same time, with a metallic conductivity type, the film becomes vulnerable for high light intensity.