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

Ronen Rapaport, Gang Chen+7 more|arXiv (Cornell University)|Aug 7, 2003

Strong Light-Matter Interactions8 references142 citations

TL;DR

This paper proposes that luminescence ring patterns in quantum well structures arise from optically-induced in-plane charge separation, where hot holes accumulate at the excitation spot due to slower cooling and drift, depleting electrons and forming a sharp electron-hole boundary. This boundary emits a persistent ring of light with sub-microsecond lifetimes, explaining long-lived emission and pattern formation without requiring indirect excitons or superfluidity.

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.

Motivation & Objective

• To explain the origin of macroscopic ring patterns in exciton luminescence observed in quantum well structures.
• To resolve the paradox of long-lived emission at large distances from the excitation spot.
• To determine whether the ring effect depends on indirect excitons or is a general phenomenon in semiconductor heterostructures.
• To identify the physical mechanism responsible for the observed ring formation and its dependence on excitation power and energy.

Proposed method

• Modeling photoexcitation above the barrier bandgap, leading to hot electron-hole pair generation in modulation-doped quantum wells.
• Simulating the dynamics of hot carriers: their cooling, trapping, and drift under applied bias.
• Accounting for the differential cooling and drift rates of electrons and holes, with holes having longer cooling times and lower mobility.
• Calculating the resulting in-plane charge separation into a hole puddle and electron sea, with a sharp interface.
• Using the product of cold electron and hole densities as the luminescence intensity profile, which peaks at the interface.
• Simulating time-resolved emission decay to match experimental persistence of ring emission.

Experimental results

Research questions

• RQ1 What causes the formation of a luminescence ring with a dark region between the excitation spot and the ring?
• RQ2 Why is the ring emission persistently long-lived, lasting over a microsecond, despite the short intrinsic exciton lifetime?
• RQ3 Why is the ring pattern observed in single quantum wells with only direct excitons, contradicting initial assumptions about indirect exciton dependence?
• RQ4 What role do hot carriers and optical depletion of electrons play in the observed macroscopic transport and pattern formation?
• RQ5 How does the ring radius scale with excitation power, and what determines the threshold for ring formation?

Key findings

• The ring pattern forms due to optically-induced in-plane charge separation, with cold holes accumulating at the excitation spot and electrons diffusing inward from the periphery.
• The ring emission arises from recombination at the sharp interface between the hole puddle and the surrounding electron sea, not from transport of pre-formed excitons.
• The ring emission persists for over a microsecond, orders of magnitude longer than the intrinsic exciton lifetime in the center spot.
• The ring radius increases sublinearly with excitation power, with a clear threshold power for ring formation.
• The ring emission spectrum remains narrow, symmetric, and unshifted, indicating a low-density, thermalized excitonic gas, while the center spot emission broadens and red-shifts with increasing power.
• Time-resolved simulations show the ring emission decays on a microsecond timescale, matching experimental observations, while the center spot decays in ~50 ps.