Transient reshaping of intersubband absorption spectra due to hot electrons in a modulation-doped multiple-quantum-well structure

The intersubband absorption spectrum of hot electrons is studied in a ${\mathrm{Ga}}_{0.47}$ ${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$ ${\mathrm{In}}_{0.52}$As multiple-quantum-well structure by picosecond infrared spectroscopy. The bandwidth of the transition between the n=1 and n=2 conduction subbands increases from 7 to 13 meV for a rise of the transient carrier temperature from 10 to 350 K. This broadening relaxes with carrier cooling on a time scale of approximately 100 ps. The different dispersion of the two subbands and the broad distribution function of hot electrons result in a temperature-dependent bandwidth of the spectrum. Theoretical calculations account quantitatively for our data.