Shcherbin K. Enhancement of the nonlinear optical response of photorefractive semiconductors in the infrared spectral range.

The thesis is addressing the actual problem of nonlinear optical response increase of photorefractive semiconductors and hybrid elements on their basis in the infrared spectral range. Analysis of the previous data shows certain advantages of CdTe and CdTe:Ge in particular. The complex study of this crystal is performed, that includes study of the photorefractive properties, optical absorption and lightinduced absorption, magnetic circular dichroism, optical detection of magnetic resonance, electron paramagnetic resonance and photo electron paramagnetic resonance. It is found that two photorefractive centers participate in the space charge formation. Energy level diagram in the bandgap is proposed for CdTe:Ge. The technique of increase of the nonlinear optical response is proposed and demonstrated, which uses optical population of necessary levels in the bandgap. The largest coupling constants for all semiconductors in diffusion mode is achieved in CdTe for the transmission geometry using the technique. The targeted search of semiconductors with large effective trap density was performed together with crystal growers from Chernivtsy National University. As a result, the largest coupling constant for all semiconductors operated in diffusion mode is achieved in contra-directional two-beam coupling in optimized crystals. Dynamic grating recording in a new photorefractive semiconductor, CdTe:Sn, is demonstrated for the first time. Tin photorefractive center is identified and characterized in the crystal using complex research. Optical excitation energies are evaluated for both charge sates of the center. The study of nonstationary photo electromotive force allows for extended characterization of the space-charge formation in the photorefractive semiconductors. Two processes operating in two different time scales are associated with two different photorefractive species in CdTe:Ge. Combining in a single hybrid device the sensitivity of the semiconductors in the infrared with large nonlinearity of the liquid crystals (LC) allows for enhanced nonlinear optical response in the infrared. Two types of hybrids are demonstrated with semiconductor substrates – photorefractive-LC hybrid and LC light valve. The best coupling constants in the infrared for such devices are reported for both designs. The generation of spatial subharmonics is achieved in a photorefractive semiconductor for the first time. Experimental study confirms the theory, which links the phenomenon with the space-charge waves. The spatial distribution of the space-charge waves is mapped using polarimetric technique. Several applications of photorefractive semiconductors with enhanced nonlinear optical response in the infrared are demonstrated. The optical limiter for infrared light using LC light valve with semiconductor substrate, all optical interconnect based on double phase conjugate mirrors in semiconductor crystals, the self-calibrated single-beam device for measurement of a transverse vibration amplitude based on photo electromotive force in semiconductor, adaptive interferometer using contra-directional two-beam coupling in photorefractive semiconductor, two-wave mixing adaptive interferometer with improved frequency response are among them.