Free-Electron Laser-Induced Bleaching of the Intersubband Absorption in Semiconductor Quantum-Wells

The intensity dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far infrared free electron laser (FELIX). Complete bleaching of the absorption is observed at I = 100 kW/cm2. When fitted with a two-level system, the saturation intensity is found to be 10 kW/CM2, which corresponds to a characteristic time constant of 1-2 ps. Possible interpretations are discussed in the situation of finite FEL pulse width.