Résumé:

Exciton recombination and spin dynamics in electrically biased semiconductor coupled quantum wells (CQWs) is dramatically slowed down by the formation of spatially indirect excitons (IX), consisting of electron and hole confined in different quantum wells. We report on time-resolved two-colors pump-probe Kerr rotation and reflectivity measurements in CQWs. Our results demonstrate that (i) Despite their vanishing oscillator strength, IXs possess remarkable nonlinear properties and induce measurable nolinear signal. (ii) IXs spin relaxation time of order of 10 ns is much longer than electron spin relaxation in the studied structure. (iii) Coherent precession of DX spin in the effective in-plane magnetic field due to linear mode splitting is washed out by the application of electrical bias: this is a direct evidence of the electron-hole exchange energy reduction.