Intersubband relaxation in GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum well structures observed directly by an infrared bleaching technique

Direct intersubband excitations in narrow, modulation doped GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum well structures are studied by picosecond infrared spectroscopy. The bleaching of the intersubband absorption induced by a high-intensity picosecond pump pulse, is studied by a delayed probe pulse. Typical intersubband relaxation times are of the order of 10 ps at 300 K for a subband splitting of about 150 meV. Polar LO-phonon scattering is discussed as the most relevant relaxation mechanism.