Time-resolved measurement of intersubband population dynamics

Summary form only given. The large spectral width of ultrashort optical pulses makes it possible to measure the complete time-resolved absorption spectrum of a sample with a single pulse, offering simultaneously high resolution in both the time and frequency domains. We have used ultrashort mid-infrared (MIR) pulses to measure the dynamic intersubband absorption spectrum of an asymmetric double quantum well excited by an ultrashort optical pulse. In our experiment an interband pump pulse injects electrons into the first and second subband of the double quantum well structure (level-spacing E/sub 21/ = 14 meV < h/spl omega//sub LO/). The time evolution of the decay of this coherent excitation into an incoherent electron population in the two subbands and their decay is monitored by probing the MIR absorption to a third (empty) subband. The MIR probe pulse is generated by phase-matched difference-frequency mixing in a 30 /spl mu/m thin GaSe crystal. The spectrum of the MIR pulse transmitted through the sample (T = 5 K) is obtained by measuring directly the electric field using a cross-correlation technique and calculating the Fourier transform.