Terahertz pulse generation in ZnTe crystal pumped around the bandgap

ZnTe is a crystal widely used to generate THz waves through optical rectification (OR) of femtosecond 800 nm laser pulses [1] . It indeed presents interesting characteristics since phase-matching is almost realized, losses are rather weak, and the birefringence issue is ignored. Its nonlinear coefficients are however quite small compared to other used crystals like LiNbO 3 for example. It should be possible to increase those nonlinear coefficients by pumping the crystal closer to its bandgap, since this enhancement has been modelled and measured for second harmonic generation [2] . Unfortunately, when dealing with THz generation, pumping at or above the bandgap excites free carriers that absorb the THz wave, leading to a decrease of THz generation efficiency. The goal of our work is to study the balance between enhanced nonlinearity and free carrier absorption by recording the THz signal versus the pump photon energy.