Influence of carrier-carrier interaction on time-dependent intersubband absorption in a semiconductor quantum well

Using ultrafast terahertz spectroscopy, we measure the temporal evolution of the intersubband absorption spectrum of a GaAs∕Al0.3Ga0.7As double-quantum-well structure with an energy spacing between the first two subbands smaller than the longitudinal optical phonon energy. We show that the interaction between the photoexcited carriers has a considerable influence on the time-dependent absorption. When varying the photoexcited sheet carrier density between 1×10¹⁰0.3em0excm^-2 and more than 1×10¹²0.3em0excm^-2, we find (i) a strong dependence of the intersubband scattering rate on the density of optically generated carriers, and (ii) a temporal shift of the intersubband resonance as the population in the second subband decays, i.e., as the photoexcited carriers relax into the quantum-well ground state.