Femtosecond dynamics of excitonic absorption in the infrared ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum wells

We investigate the exciton ionization process in ${\mathrm{In}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum wells using 100-fs time-resolved spectroscopy. The ionization time is studied as a function of temperature. We find a room-temperature ionization time of ≊200 fs and longer times at lower temperatures. At room temperature the ionization is dominated by collisions with LO phonons, whereas at lower temperatures, another extrinsic mechanism becomes increasingly important. We also measure the effect of free electrons and holes versus excitons on the excitonic absorption. We find that excitons are ∼3 times more efficient at room temperature, and that this ratio decreases with temperature. This value is larger than that measured for GaAs. The result is compared with the theory which predicts scaling of the ratio with temperature and the inverse of the exciton binding energy. Good qualitative agreement is obtained.