Far-Infrared harmonic generation from semiconductor heterostructures

Carriers confined in quantum well structures in the GaAs-Al_xGa_1-xAs 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^-1 - 25 cm^-1 for the millimeter FEL, and 32 cm^-1 to 155 cm^-1 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, LiNbO₃.