Biexponential intersubband relaxation in n-modulation-doped quantum-well structures

Intersubband scattering in an n-modulation-doped GaAs/Al0.31Ga0.69As quantum-well structure is systematically investigated as a function of temperature and pump intensity. For the first time biexponential relaxation is observed during infrared bleaching experiments. The fast component with a time constant of similar to 1 ps, which represents the depopulation of the first excited well subband, is found to dominate the signal more and more with decreasing temperature and pump intensity. The decay time of the slower component rises with decreasing temperature from tau(2) = 8 ps at T = 300 K to tau(2) = 23 ps at T = 10 K. This component is believed to be connected with a carrier transfer to the potential minima in the barrier layers. The intensity dependent excitation mechanism and the relaxation processes are discussed with the help of detailed numerical simulations. (C) 1996 Academic Press Limited