Time-resolved, nonperturbative, and off-resonance generation of optical terahertz sidebands from bulk GaAs

We have investigated terahertz (THz) sideband generation from bulk GaAs using intense pulses of coherent THz (or far-infrared) radiation from a free electron laser. In contrast to previous studies, sidebands appeared inside the band gap and therefore were not resonantly enhanced from real states. Also, using picosecond pulses and changing the temporal overlap between the THz and near-infrared pulses allowed us to monitor the evolution of the THz sidebands directly in the time domain; this suggests a convenient method for characterizing THz pulses using a conventional Si photodetector. In addition to an expected second-order sideband, we detected a first-order sideband, which has previously been observed only in an asymmetrically coupled double quantum well system where the inversion symmetry was intentionally broken. Finally, the THz power dependence clearly revealed a deviation from perturbative behavior, indicating the entrance into the strong-field regime.