Far-Infrared Low-and High-Order Harmonic Generation in GaAs/AlGaAs Quantum Wells.

Abstract : We have used the UCSB Free-Electron Lasers to study nonlinear and nonequilibrium intersubband dynamics in GaAs/AlGaAs semiconductor heterostructures at Terahertz frequencies. We have observed extremely large voltage-tunable resonant optical nonlinearities in quantum wells, which can be successfully modeled by self-consistent theory which includes the electron-electron interactions. We have performed definitive measurements of the intersubband relaxation time for quantum wells with energy level spacings below the LO phonon energy. Finally, we have observed the collective intersubband absorption in a quantum well to shift from its 'dressed' energy towards its undressed energy on excitation with intense Terahertz radiation. jg p1