Hole relaxation in p-type ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As quantum wells observed by ultrafast midinfrared spectroscopy

We have performed midinfrared pump-probe transmission measurements in p-type strained ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As quantum wells at room temperature. The laser source was a midinfrared free-electron laser generating subpicosecond pulses and tuned on resonance between the heavy-hole and the light-hole subbands. The relaxation time is ∼1 ps, which is shorter than for electrons in the conduction band. Interactions between the optical phonons and holes account for the measured relaxation.