Ultrafast terahertz spectroscopy can be used to probe charge and spin dynamics in semiconductors. We have studied THz emission from bulk InAs and GaAs and from GaAs/AlGaAs quantum wells as a function of magnetic field. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-Sapphire laser, and we recorded the THz emission spectrum and the integrated THz power as a function of magnetic field and temperature. In bulk samples the emitted radiation is produced by coupled cyclotron-plasma oscillations: we model THz emission from n-GaAs as magneto-plasma oscillations in a 3-D electron gas. THz emission from a modulation-doped parabolic quantum well is described in terms of coupled intersubband-cyclotron motion. A model including both 3-D plasma oscillations and a 2-D electron gas in a surface accumulation layer is required to describe THz emission from InAs in a magnetic field.
Carriers confined in quantum well structures in the GaAs-Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As and InAs-AlSb systems have strongly nonlinear response at far-infrared (FIR) frequencies: they are strong harmonic generators. To date, studies of FIR frequency harmonic generation from heterostructures have been limited primarily to discrete frequencies accessible by molecular gas lasers (MGL). Because the mechanisms responsible for harmonic generation are expected to have frequency dependence in the FIR, the broad, continuous frequency coverage of the free electron lasers (FEL) at UCSB (4 cm<SUP>-1</SUP> - 25 cm<SUP>-1</SUP> for the millimeter FEL, and 32 cm<SUP>-1</SUP> to 155 cm<SUP>-1</SUP> for the FIR FEL) make them ideal sources for studies of free carrier nonlinearities in heterostructures. Quantitative experiments on harmonic generation must distinguish the small harmonic signals generated by the samples from the fundamental, and from the harmonic content of the incident FEL light. Here we discuss strategies for achieving the required selectivity, preliminary data taken with an MGL on heterostructure samples, and data taken with the FEL for a suitable calibration sample, LiNbO<SUB>3</SUB>.