Picosecond infrared spectroscopy of hot carriers in a modulation-doped ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As multiple-quantum-well structure

Intersubband scattering and cooling of hot carriers is investigated in an $n$-type modulation-doped ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$ ${\mathrm{In}}_{0.52}$As multiple-quantum-well structure. The nonlinear absorption is monitored in several picosecond pump-and-probe experiments after selective excitation of the electrons to the $n=2\mathrm{}$ subband. In addition, the relaxation of hot electron-hole pairs created by interband absorption is studied. An upper limit of 3 ps for the lifetime of the electronic $n=2\mathrm{}$ subband is estimated. The cooling is dominated by LO-phonon scattering; the energy loss rate is reduced by a factor of up to 130 compared to the value obtained from theoretical calculations.