Picosecond CO2laser-induced self-defocusing in InSb

Self-defocusing of 45 ps CO
2
laser pulses was observed in InSb at intensities as low as 1 MW/cm
²
. This self-defocusing was caused by the two-photon absorption generated free carriers and the resulting self-phase modulation. In addition to demonstrating spatial beam modulation in this material at 10.6 μm, the present experiment also confirms directly that such a process occurs on a picosecond time scale. This information is important in order to increase the switching speed of infrared optical bistable devices to the picosecond regime. Furthermore, the present self-defocusing experiment also provided an accurate determination of the two-photon absorption coefficient in InSb. The value of 2.5 cm/MW obtained agrees exactly with a previous study. Other applications to continuum and ultrashort pulse generation will also be discussed.