Intersubband dynamics of asymmetric quantum wells studied by THz 'optical rectification'

We have measured the rectification of far-infrared radiation resonant with the lowest intersubband transition of an AlGaAs/GaAs asymmetric coupled double-quantum well in which the subband spacing is 11 meV. From these measurements we can extract an intersubband lifetime of 1.2 +/- 0.4 ns at low excitation intensity and T = 10 K, which appears promising for devices such as FIR detectors or mixers which can operate at low excitation and temperature. In order to investigate the effect of carrier concentration on the relaxation time we have performed the same experiments in a logarithmically graded quantum well.