We experimentally investigate the lateral diffusion of dipolar excitons in coupled quantum wells in two (2D) and one (1D) dimensions. In 2D, the exciton expansion obeys nonlinear temporal dynamics due to the repulsive dipole pressure at a high exciton density, in accordance with recent reports. In contrast, the observed 1D expansion behaves linearly in time even at high exciton densities. The corresponding 1D diffusion coefficient exceeds the one in 2D by far and depends linearly on the exciton density. We attribute the findings to screening of quantum well disorder by the dipolar excitons.

We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum well efficiently traps dipolar excitons and leads to the stabilization of the excitonic formation and confinement in the illumination area. Hereby, the density of dipolar excitons is remarkably enhanced up to $\sim 10^{11} cm^{-2}$. By means of Landau level spectroscopy we study the density of excess holes in the illuminated region. Depending on the excitation power and the applied electric field, the hole density can be tuned over one order of magnitude up to $\sim 2.5$ $10^{11} cm^{-2}$ - a value comparable with typical carrier densities in modulation-doped structures.Comment: 4.3 Pages, 4 Figure

Spatially indirect excitons in a coupled quantum well structure were studied by means of polarization and time resolved photoluminescence. A strong degree of circular polarization (> 50%) in emission was achieved when the excitation energy was tuned into resonance with the direct exciton state. The indirect transition remained polarized several tens of nanoseconds after the pumping laser pulse, demonstrating directly a very long relaxation time of exciton spin. The observed spin memory effect exceeds the radiative lifetime of the indirect excitons. PACS numbers: 78.47.jd, 78.67.De Semiconductor heterostructures with tunable spatial separation of electrons and holes attract a great attention for their possible applications in optoelectronic devices [1-4]. While the storage of excitons for several microseconds has been experimentally evidenced [1,[5][6][7][8], it is still an open question how to exploit the spin of indirect excitons for information storage. Recent experiments performed on coupled quantum wells (CQWs) reported exciton spin transport in the range of micrometers, suggesting indirectly a long exciton spin relaxation time of nanoseconds [9]. These results suggest that a local probe can overcome the spin relaxation caused by the spin propagation dynamics. Here, we exploit a confocal optical scheme in order to study the spin dynamics of indirect excitons at the location of their excitation. Hereby, we resolve the spin memory of indirect excitons in coupled quantum wells via time-and polarization-resolved PL studies after resonant and non-resonant excitation of the direct and indirect exciton states. We observe that under conditions of resonant excitation a highly efficient initialization of exciton spin takes place. The PL remains strongly circularly polarized long after the laser pulse and nearly constant during the lifetime of excitons. Our results directly confirm a long spin relaxation time of > 80 nanoseconds for indirect excitons. This time scale is an order of magnitude longer than the one obtained by readout schemes with a larger focus spot [10].The studied heterostructure consists of two 8-nm wide GaAs/AlGaAs coupled quantum wells separated by a 4nm Al 0.3 Ga 0.7 As barrier and it was fabricated as a fieldeffect device [11][12][13]. The shape of the confining potential is controllably adjusted by the bias (V g ) applied between a semitransparent metal gate and a deep ohmic contact, allowing the direct manipulation of the exciton lifetime via a modified electron-hole separation (Fig. 1 (a)). The PL excitation and collection are performed through the semitransparent part of the top Schottky gate using a confocal microscope based on a short focal length aspheric lens. The luminescence is dispersed by a 0.5 m double monochromator and detected with an intensified charge coupled device (ICCD) detector. The configuration of the setup allows us to perform measurements under identical ("co") and orthogonal ("cross") polarizations of excitation and detection. The resulting extinction ratio is below 0.00...

We study the influence of lithographically defined, electrostatic trap configurations on the photon emission from dipolar excitons in coupled quantum wells. The emission is surprisingly enhanced for an excitonic antitrap compared to a trap configuration, an effect more pronounced for a trap with smaller diameter. We explain the observations by the interplay between the exciton formation process, the lateral charge-carrier dynamics, and the dipole-dipole interactions between the excitons. Exploiting this interplay allows us to efficiently tune the excitonic emission energy with very small intensity variation.