SATURATION OF INTERSUBBAND ABSORPTION BY REAL-SPACE TRANSFER IN MODULATION-DOPED SINGLE GAAS-ALAS QUANTUM-WELL

Intersubband absorption spectrum was investigated with intense infrare d light in a modulation doped GaAs quantum well (QW) of 10 nm thicknes s with 6 nm AlAs barriers. When the excitation intensity was 9.8 kW/cm (2), the absorption was bleached to half of its weak field value. The correspending effective intersubband relaxation time tau was 14 ps, wh ich is much larger than the intersubband LO phonon emission time. We a lso observed that the peak position energy E-p shifts toward lower pho ton energy as the excitation intensity is increased, however, the amou nt of the shift was about a factor four smaller than the value predict ed from the depolarization effect. The observed large tau and the smal l shift of E-p indicates an efficient real-space transfer of the excit ed electrons. The escape of electrons was directly detected as a photo current.