Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

Abstract

Indirect excitons in coupled quantum wells are long-living quasi-particles, explored in the studies of collective quantum states. We demonstrate, that despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons, but also one of free electrons by two orders of magnitude.

Influence of magnetic quantum confined Stark effect on the spin lifetime of indirect excitons

“…We have shown in our previous work, that the spin dynamics of DXs, IXs, and residual 2DEG in coupled quantum wells can be efficiently addressed in this configuration [13]. Figure 2 presents the main result of this work.…”

“…In this regime, IX is the lowest energy exciton state of the system and in the Kerr rotation signal at B = 0 we observe three exponentially decaying components (squares in Fig. 3 (a)) [13]. They can be attributed to the spin relaxation of the DX, 2DEG, which forms in biased CQWs [25][26][27], and IXs.…”

“…We have shown in our previous work that at low excitation energy and power, one can reach the regime where exciton spin precesses even in the absence of the applied magnetic field [13]. This precession is due to a small splitting between two perpendicularly polarized linear exciton states δ xy that is generally present in QW structures [18,19].…”

“…For pure IX the exchange interaction is negligible due to the low electronhole overlap, so that the fluctuating wave-vector dependent exchange field does not affect spin relaxation. At low exciton densities, IX spin lifetime can be even longer than the spin relaxation time of a two-dimensional electron gas (2DEG) systematically present in biased QWs [13]. This difference is essentially due to stronger localisation of IXs in the QW disorder potential, as compared to 2DEG or DX.…”

“…This difference is essentially due to stronger localisation of IXs in the QW disorder potential, as compared to 2DEG or DX. The variation of the spin lifetime of excitons with the gate voltage can be understood as being due to the mixing between the purely DX state characterized by a fast relaxation rate, and the purely IX state having a slow spin relaxation rate [13].…”