Résumé:

A spatially indirect exciton (IX) can be formed when an electron and a hole are confined in two different spatially separated coupled quantum wells (CQWs). The spatial separation of an electron and a hole wavefunctions can be engineered to control the spin lifetimes of IXs. In GaAs/AlGaAs CQWs, IX spin relaxation time up to tens of nanoseconds have been recently reported [1,2]. However, the measurements of IX spin dynamics by nonlinear spectroscopy are challenging, due to low oscillator strength of the IX state. We demonstrate a proof-of-concept for time-resolved pump-probe spectroscopy of IXs in CQWs. Pump and probe light pulses are resonant with optically active direct exciton (DX) transition, and IXs are probed via their common ground state with DXs (Fig.1a). Photoinduced reflectivity measurements show that the IX lifetime time in biased CQWs is of order of 30 ns at Vg=0.8 V, consistent with the PL kinetics measurments [3]. Pump-probe experiments allow unraveling DX, IX and electron spin dynamics. It appears that not only DX, but also bare electron spin relaxation is much faster (≃ 200 ps) than one of IXs (up to 10 ns).