Abstract

We report the first direct measurement of the relaxation time of holes in p-type quantum wells using femtosecond mid-infrared laser pulses tuned on resonance with the intersubband transition energy. In the early experiments, we used the Stanford free electron laser (FEL) which generated submicrojoule, subpicosecond pulses tunable from 4 to 6 μm [1]. Recently, we have used microjoule femtosecond pulses tunable from 4 to 9 μm generated by frequency mixing the signal and idler outputs of a parametric amplifier pumped by an amplified Ti:S laser. Most of the measurements have been performed in the degenerate pump-probe configuration as a function of hole density, sample temperature, pump wavelength and pump intensity. The multiple quantum well (MQW) samples investigated to date consist of strained In_0.5Ga_0.5As wells and Al_0.5Ga_0.5As barriers. Fig. 1 shows the infrared absorption of a typical sample containing a hole sheet density of 10¹³ cm^-2. The absorption at 5.25 μm corresponds to the n=1 heavy hole (hh) to n=1 light hole (lh) transition.

© 1996 Optical Society of America