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 E(13) and E(23) at 10 and 14.5 mu m, respectively. To measure the intersubband relaxation time in the E(3) 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 mu m and pumps the E(13) intersubband transition while the second colour (14.5 mu m) probes the E(23) intersubband transition. A relaxation time of similar to 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.