The effect of composition on photoconductivity and nonlinear optical properties in the acentric Ag2In2AB6 (A = Si, Ge, B = S, Se) crystals

Abstract

Complex studies of the photoconductivity, photoinduced second harmonic generation (SHG) and linear electrooptic for four quaternary chalcogenide crystals Ag2In2GeSe(S)6 and Ag2In2SiSe(S)6 that crystallize in the monoclinic structure have been done in the infrared spectral range. As a photoinduced beam we have used a bicolor coherent 9400/4700 nm laser beam of microsecond pulsed CO2 laser. The electrooptical effect was probed by continuous wave He-Ne laser at wavelength 3390 nm and second harmonic generation by nanosecond pulse Er:glass laser at wavelength 1540 nm. The relation between the energy gap, output electrooptic efficiency and the photo stimulated SHG has been done. The detailed kinetics of the photoconductivity show that there occurs significant re-distribution of photoexcited carriers between the defects. This factor also may play a crucial role both for the photoinduced electrooptic as well as for second harmonic generation. It was experimentally shown that there exists an increase of the second-order susceptibility with enhanced electronic energy gap which contradicts compared to the generally adopted approach. Such extraordinary behavior is a consequence of the significant contribution of anharmonic phonons in the infrared spectral range spectrally situated near the nonlinear optical resonances. Additional role here may also belong to the space re-distribution of carriers between the defects that was established following the photoconductivity kinetics studies.