Many excitations of semiconductors and their nanostructures lie in the Terahertz range of the electromagnetic spectrum. These excitations include transitions between bound states of shallow impurities, intersubband transitions in wide quantum wells, and intraband transitions in quantum dots. Although linear spectroscopy of these excitations has been studied for many years, studies of nonlinear response and relaxation of these excitations towards equilibrium has been hampered by the absence of suitable sources of radiation. UCSB’s Free-Electron Lasers enable such studies for photon energies ranging from roughly 0.5 meV to 20 meV. The GaAs/AlGaAs system is a particularly clean and rich system for such nonlinear and nonequilibrium studies, and is also a leading candidate for new THz devices such as sources and detectors of THz radiation. New semiconductor physics which can be addressed with THz driving includes
* Relaxation of THz intraband excitations in quantum wells, quantum dots, and impurity states.
* Dynamics and dissipation of collective modes, like intersubband excitations.
* Quantum coherent dynamics of electrons bound to shallow impurities.
* Changing band structure with intense THz radiation.