Suppression of exciton super-radiance due to spatially separated scatterers

We have found that the radiative lifetime of an exciton in a GaAs quantum well (QW) increases due to high-density excitons injected in an exciton reservoir spatially separated from the QW. The photoinjected excitons in the reservoir interact with the two-dimensional exciton in the QW via the Coulomb interaction without the phase-space filling effects and the exchange interaction. The radiative lifetime increases in proportion to the exciton density in the reservoir initially, while it saturates for the higher density. It is interpreted as suppression of the exciton super-radiance due to weak localization of the exciton center-of-mass motion caused by the excitons in the reservoir. A phenomenological two-level model reproduces the experimental observation, where an exciton state is represented as the scattering-induced superposition of small wave-vector (radiative) and large wave-vector (nonradiative) states.