Abstract
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/cm
2. When fitted with a two-level system, the saturation intensity is found to be 10 kW/cm
2, which corresponds to a characteristic time constant of 1–2 ps. Possible interpretations are discussed in the situation of finite FEL pulse width.