We have investigated terahertz ͑THz͒ sideband generation from bulk GaAs using intense pulses of coherent THz ͑or far-infrared͒ 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.

A promising method for studying intraband carrier dynamics in semiconductors is monitoring the evolution of far-infrared ͑FIR͒ absorption induced by photoexcited carriers. By monitoring the photoinduced FIR absorption as a function of magnetic field, we performed time-resolved cyclotron resonance of photocreated electrons in InSb with picosecond resolution.

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A comparison of impurity-free and ion-implantation-induced intermixing of InGaAs/InP quantum wellsUsing synchronized near-infrared ͑NIR͒ and terahertz ͑THz͒ lasers, we have performed picosecond time-resolved THz spectroscopy of transient carriers in semiconductors. Specifically, we measured the temporal evolution of THz transmission and reflectivity after NIR excitation. We systematically investigated transient carrier relaxation in GaAs and InSb with varying NIR intensities and magnetic fields. Using this information, we were able to determine the evolution of the THz absorption to study the dynamics of photocreated carriers. We developed a theory based on a Drude conductivity with time-dependent density and density-dependent scattering lifetime, which reproduced the observed plasma dynamics. Detailed comparison between experimental and theoretical results revealed a linear dependence of the scattering frequency on density, which suggests that electronelectron scattering is the dominant scattering mechanism for determining the scattering time. In InSb, plasma dynamics was dramatically modified by the application of a magnetic field, showing rich magnetoreflection spectra, while GaAs did not show any significant magnetic field dependence. We attribute this to the small effective masses of the carriers in InSb compared to GaAs, which made the plasma, cyclotron, and photon energies all comparable in the density, magnetic field, and wavelength ranges of the current study.

Using intense picosecond pulses of coherent terahertz (THz) radiation, we have investigated time-resolved non-resonant THz sideband generation from bulk GaAs. The THz power dependence clearly reveals a non-perturbative strong-field regime. In addition to the expected ω−2 sideband, we detected the ω−1 (or odd) sideband which has previously been observed only in a quantum well system where the inversion symmetry was intentionally broken.