Abstract

Laser-induced gratings produced by one- and two-photon absorption of picosecond pulses at 1.064 μm were used to examine the room temperature nonlinear optical responses of undoped n- and p-type CdTe crystals. We report what we believe to be the first observation of picosecond photorefractive two-beam coupling in a material that has no cw response. Gains up to 2.5 cm⁻¹ were obtained at fluences of 120 mJ cm⁻² for 0.8 μm grating spacing. Polarization rotation switching was also measured with on/off ratios of 6.0 at fluences of 56 mJ cm⁻². Pulse- probe degenerate four-wave mixing measurements of grating dynamics on subnanosecond time scales were used to measure the ambipolar diffusion coefficient of charge carriers in the crystals. The value of D_α = 3.0 cm² s⁻¹ which was obtained is in very good agreement with theoretical estimates. Free carrier lifetimes of 12 ns were measured. A long-lived contribution to the signal consistent with a trapped charge photorefractive effect was observed at large grating spacings for n-type conductivity, and is tentatively attributed to a larger trap density in this sample. Measurements of the relative scattering efficiencies of successive diffracted orders in the Raman-Nath regime allowed for calculation of the laser-induced change in the index of refraction, Δn due to the creation of free carriers. The value of Δn = 4 x 10⁻⁴ which was obtained is in good agreement with theoretical estimates. ^*Research sponsored by DARPA contract MAD972-89-K0001.

© 1990 Optical Society of America