Abstract

The role of nonlinear optical phenomena in the generation of terahertz radiation via nonlinear mixing of ultrashort infrared laser pulses at semiconductor surfaces is described. Several mechanisms lead to the generation of terahertz radiation from semiconductor surfaces, including depletion-field driven current (current surge),1 bulk difference-frequency mixing (DFM),² and electric-field-induced optical rectification.3 Previous studies reported that when InSb is illuminated with unfocused above-gap ultrashort optical pulses, the spectrum of the emitted radiation, which is due to the current surge mechanism, shifted to lower frequency as the sample temperature decreased.¹,⁴ This unique feature has led us to study the temperature dependence of difference-frequency mixing from InSb crystals illuminated with focused laser pulses. Increasing the intensity of the 100 fs laser pump beam leads to measurable amounts of terahertz radiation by nonlinear mixing of the spectral components of the pump pulse, as well as a decrease in the current-surge-induced terahertz radiation.² By rotating an undoped (111) InSb sample about its surface normal, we were able to separate the contribution of the azimuthally-dependent DFM signal from the total terahertz waveform. Contrary to the current surge pulses emitted from (111) InSb, the DFM pulses did not show any frequency shift with temperature. However, there was a pronounced temperature dependence of the DFM pulse amplitude that closely followed that seen from the current surge pulses. Finally, analysis of the angular dependence of terahertz radiation emitted from (100) InAs shows that the dominant nonlinear mixing term is bulk optical rectification.

© 1996 Optical Society of America