Direct observation of intersubband relaxation dynamics and application to subpicosecond all-optical modulation

An improved all-optical modulation scheme based on intersubband transition in n-doped quantum well is proposed, which utilizes electron population dynamics in the 2nd conduction subband instead of the 1st subband utilized in the previous scheme. The modulation characteristics and the intersubband relaxation dynamics are investigated by white-light pump-probe spectroscopy in GaAs/AlGaAs quantum wells. The response time of the modulation is found to be ~0.4 ps, which is much shorter than theoretically expected value of ~1 ps. It is also found that the population relaxation time in the 1st subband is longer than that in the 2nd conduction subband as a factor of 10. The discrepancies are attributed to the existence of the relaxation path via subbands in L valley that has very large density of states