Saturation of Intersubband Absorption by Real-Space Transfer in Modulation Doped Single GaAs–AlAs Quantum Well

Intersubband absorption spectrum was investigated with intense infrared light in a modulation doped GaAs quantum well (QW) of 10 nm thickness with 6 nm AlAs barriers. When the excitation intensity was 9.8 kW/cm, the absorption was bleached to half of its weak field value. The corresponding effective intersubband relaxation time t was 14 ps, which is much larger than the intersubband LO phonon emission time. We also observed that the peak position energy Ep shifts toward lower photon energy as the excitation intensity is increased, however, the amount of the shift was about a factor four smaller than the value predicted from the depolarization effect. The observed large t and the small shift of Ep indicates an efficient real-space transfer of the excited electrons. The escape of electrons was directly detected as a photocurrent.