Excite-Probe Determination of the Intersubband Lifetime in Wide Gaas/Algaas Quantum-Wells Using a Far-Infrared Free-Electron Laser

A direct excite-probe semiconductor lifetime determination in the picosecond regime has been made for the first time in the far infrared. We have used an RF-linac-pumped free-electron laser to determine the relaxation rate associated with intersubband absorption in GaAs/AlGaAs quantum wells having a subband separation smaller than the optical phonon energy. The measurement yields a relaxation lifetime of 40 +/- 5 ps. This is compared with a variety of other results obtained with less direct techniques.