Time resolved measurement of intersubband lifetime in GaAs quantum wells using a two-colour free electron laser

The authors have studied the intersubband relaxation in asymmetric GaAs/AlGaAs coupled quantum wells using a two-colour free electron laser. Three subbands are bound in the conduction band and exhibit two intersubband transitions in the mid-infrared E₁₃ and E₂₃ at 10 and 14.5 μm, respectively. To measure the intersubband relaxation time in the E₃ subband, a time-resolved pump and probe experiment is performed in multipass waveguide geometry with the picosecond two-colour free electron laser facility CLIO in Orsay. At room temperature, the first colour is set at 10 μm and pumps the E ₁₃ intersubband transition while the second colour (14.5 μm) probes the E₂₃ intersubband transition. A relaxation time of ~0.5 ps on the third subband is measured. This value is in agreement with theoretical relaxation calculations which take into account interface and slab phonon modes