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

TL;DR

This paper explains the formation of luminescence ring patterns in quantum well structures through a novel in-plane charge separation mechanism, supported by experimental observations and quantitative modeling, revealing long-lived exciton dynamics.

Contribution

It demonstrates that ring patterns occur in single quantum wells and are explained by coupled electron-hole plasma dynamics, introducing a new understanding of nonequilibrium pattern formation.

Findings

Ring patterns observed in single quantum wells with only direct excitons.

Long luminescence times exceeding a microsecond due to charge separation.

Quantitative explanation of patterns via coupled 2D electron-hole plasma dynamics.

Abstract

About a year ago, two independent experiments [1,2], imaging indirect exciton luminescence from doped double quantum wells under applied bias and optical excitation, reported a very intriguing observation: under certain experimental conditions, the exciton luminescence exhibits a ring pattern with a dark region in between the center excitation spot and the luminescent ring that can extend more than a millimeter from the center spot. Initial speculations on the origin of this emission pattern included supersonic ballistic transport of excitons due to their dipole-dipole repulsion and Bose superfluidity of excitons. In this paper we show that the ring effect is also observed in single quantum well structures, where only direct excitons exist. More importantly, we find that these experimental results are quantitatively explained by a novel coupled 2D electron-hole plasma dynamics, namely, photoinduced in-plane charge separation. This charge separation explains extremely long luminescence times that may be more than a microsecond for the ring -- orders of magnitude longer than the emission lifetime of the excitons in the center spot. This method of continuously creating excitons may result in a highly dense exciton gas which is also well thermalized with the lattice (since the particles can cool over the very long luminescence time after their hot optical creation), thus opening up opportunities for a detailed study of quantum statistics. The in-plane separation of the charges into positive and negative regions, with a sharp interface between them is an interesting new example of nonequilibrium dynamics and pattern formation.