An experiment conducted to resolve the puzzling observation of large diameter exciton emission halos around a laser excitation spot in two dimensional systems was discussed. The ring emission persisted for an extremely long time compared to the center spot lifetime after the laser excitation was turned off due to the macroscopic charge separation. The ring time was determined by a slow carrier diffusion, driven by the carrier density gradients in the plane and the center spot lifetime was determined by the free carrier recombination time. The charge separation dwell in the charge reservoirs allowed for cooling of initially hot optically generated carriers.
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This work has been partially supported by the National Science Foundation and the Department of Energy. We thank Phil Platzman, Girsh Blumberg, Nikolai Zhitenev, Rafi de Picciotto, Chandra Varma, and Peter Littlewood for helpful discussions.