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.

GaN/AlGaN polar single quantum wells have attracted particular attention due to their large binding energies and the presence of strong internal electric fields that naturally induce long-lived excitons. In this case, the exciton transport is driven by the potential landscape caused by the local alteration of the built-in electric field, which results from the exciton accumulation at the same point. We study by spatially- and time-resolved PL experiments AlGaN/GaN coupled quantum wells (QW). The active zone of studied sample consists of a 30 nm thick Al0.20Ga0.80N barrier, a 4.4 nm (17 atomic monolayers) GaN QW followed by 1 monolayer of Al0.20Ga0.80N and a 1.6 nm (6 atomic monolayers) GaN QW and 30 nm thick Al0.20Ga0.80N cap layer. By solving Schroedinger-Poisson equation, we illustrate the two regimes of strongly screened electric field and the totally un-screened QW system. We show that the screening of the build-in electric field enhances the electron-hole wave-function overlap and increases the optical recombination energy. In other words, dipoledipole repulsion between excitons raises their potential energy and drastically increases their radiative recombination rate. A blue-shift of 0.3 eV under continues wave, localized excitation and a quasi-continuous red-shift of the emission as excitons are drifted away from the excitation spot up to distances of 90 μm are observed experimentally. Strong LO-phonon replica are also noticed, as a result of the strongly dipolar character of excitons [1]. These replicas follow the same position dependence as the zerophonon line. Using our Schroedinger-Poisson equation solver, we can extract from the measured blue-shift the position-dependent exciton density. Last, we present simulations of the observed transport features, namely the exciton densities and luminescence intensities by solving the nonlinear drift-diffusion equation. We establish that the long-range excitonic transport is mainly dominated by drift caused by repulsive dipole-dipole interaction between excitons in combination with their long lifetimes. [1] S. Kalliakos, X. B. Zhang, T. Taliercio, P. Lefebvre, B. Gil, N. Grandjean, B. Damilano, and J. Massies. Applied Physics Letters 80, 428 (2002).

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).

Semiconductor coupled quantum wells (CQWs) offer an excellent laboratory for studying both intra-well direct excitons (DX) and inter-well indirect excitons (IX), as well as their interactions and spin dynamics. The presentation will focus on the pump-probe spectroscopy, a powerful tool of nonlinear optics, that we have applied to biased CQW. We will show how DX and IX spin and population dynamics, as well as the spin polarization of residual electrons may be detected via the modulation of reflectivity and Kerr rotation spectra . In particular, the origin of the nonlinearities related to the low oscillator strength IXs will be addressed. Finally, a brief discussion of the spin relaxation mechanisms for DX, IX and electron gas will be presented

Electromagnetic diffraction fromnanostructured objects: numerical challenges