Picosecond nonlinear optical phenomena in undoped cadmium telluride

Laser-induced gratings produced by two-photon absorption of picosecond pulses at 1.064 μm were used to examine the room temperature nonlinear optical responses of undoped CdTe crystals with different types of conductivity. We report what we believe to be the first observation of photorefractive two-beam coupling and transient energy transfer owing to space charge fields produced by electron-hole separation. Gains up to 2.5 cm
^-1
were obtained at fluences of 12 mJ cm
^-2
for 1.9 μm grating spacing. Good qualitative agreement is shown between the experimental results and theory. Scalar polarization rotation switching was also measured with on/off ratios of 6 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 Da = 3.0 cm
²
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. The contribution of optical second harmonic generation (SHG) to the generation of free carriers in thick samples of CdTe was measured. We find the ratio of the number of photons generated by SHG to that generated by all two-photon absorption processes to be about 4 %, demonstrating that the contribution of this mechanism to free carrier generation and therefore to the measurement of the two-photon absorption coefficient is not insignificant. 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, △n, due to the creation of free carriers. The value of △n = 4 x 10-4 which was obtained is in good agreement with theoretical estimates.