Nonlinear-process-induced period doubling of a picosecond transient grating in CdS.

Abstract

The occurrence of higher-order Fourier components in an originally sinusoidal free-carrier index grating produced in CdS by the interference of two picosecond light pulses is demonstrated by monitoring simultaneously the temporal behavior of the first- and second-order diffraction intensities of transparent probe pulses for the temperature range 10\char21{}140 K. Nonlinear processes which take part in the decay of photoexcited carriers result in a deformation of the shape of the grating, which leads to the occurrence of higher-order Fourier components in the diffraction grating. Theoretical analysis of the experimental results yields the following distinct processes: bimolecular recombination of electrons and holes for the degenerate plasma, free-carrier\char21{}to\char21{}exciton Mott transition for the nondegenerate plasma at low carrier temperatures, and a nonlinear relation between exciton density and free-carrier density at high temperatures.