Mechanism of Luminescence Ring Pattern Formation in Quantum Well Structures: Optically-Induced In-Plane Charge Separation

摘要

About a year ago, two independent experiments [1,2], imaging indirect excitonluminescence from doped double quantum wells under applied bias and opticalexcitation, reported a very intriguing observation: under certain experimentalconditions, the exciton luminescence exhibits a ring pattern with a dark regionin between the center excitation spot and the luminescent ring that can extendmore than a millimeter from the center spot. Initial speculations on the originof this emission pattern included supersonic ballistic transport of excitonsdue to their dipole-dipole repulsion and Bose superfluidity of excitons. Inthis paper we show that the ring effect is also observed in single quantum wellstructures, where only direct excitons exist. More importantly, we find thatthese experimental results are quantitatively explained by a novel coupled 2Delectron-hole plasma dynamics, namely, photoinduced in-plane charge separation.This charge separation explains extremely long luminescence times that may bemore than a microsecond for the ring -- orders of magnitude longer than theemission lifetime of the excitons in the center spot. This method ofcontinuously creating excitons may result in a highly dense exciton gas whichis also well thermalized with the lattice (since the particles can cool overthe very long luminescence time after their hot optical creation), thus openingup opportunities for a detailed study of quantum statistics. The in-planeseparation of the charges into positive and negative regions, with a sharpinterface between them is an interesting new example of nonequilibrium dynamicsand pattern formation.