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 $\mathrm{GaAs}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ 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 $1\times {10}^{10}{\mathrm{cm}}^{-2}$ and more than $1\times {10}^{12}{\mathrm{cm}}^{-2}$, 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.