[论文解读] 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 Interactions参考文献 8被引用 142

一句话总结

本文提出，量子阱结构中的发光环状图案源于光致诱导的面内电荷分离，其中热空穴由于冷却和漂移速度较慢而在激发点处累积，导致电子耗尽并形成清晰的电子-空穴边界。该边界发射出持续时间达亚微秒量级的环状光，解释了长寿命发光与图案形成现象，而无需依赖间接激子或超流性。

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.

研究动机与目标

• 解释在量子阱结构中观测到的宏观环状图案在激子发光中的起源。
• 解决在激发点远处仍存在长寿命发光的悖论。
• 确定环状效应是否依赖于间接激子，或是否为半导体异质结构中的普遍现象。
• 识别导致观测到的环状结构形成的物理机制，及其与激发功率和能量的关系。

提出的方法

• 建模高于势垒带隙的光激发，导致调制掺杂量子阱中产生热电子-空穴对。
• 模拟热载流子的动力学行为：包括其冷却、捕获及在外加偏压下的漂移。
• 考虑电子与空穴在冷却和漂移速率上的差异，其中空穴具有更长的冷却时间与更低的迁移率。
• 计算由此产生的面内电荷分离，形成空穴液滴与电子海，并在两者间形成清晰界面。
• 使用冷电子与空穴密度的乘积作为发光强度分布，其峰值出现在界面处。
• 模拟时间分辨发光衰减，以匹配实验中观察到的环状发光持久性。

实验结果

研究问题

• RQ1 是什么导致了在激发点与环之间存在暗区的发光环状图案的形成？
• RQ2 为何环状发光持久存在超过一微秒，尽管其本征激子寿命极短？
• RQ3 为何在仅含直接激子的单量子阱中也观测到环状图案，这与最初假设的间接激子依赖性相矛盾？
• RQ4 热载流子及电子的光致耗尽在观测到的宏观输运与图案形成中起何种作用？
• RQ5 环半径如何随激发功率变化，环形成的阈值由什么决定？

主要发现

• 环状图案源于光致诱导的面内电荷分离，冷空穴在激发点处累积，而电子则从外围向内扩散。
• 环状发光源于空穴液滴与周围电子海之间清晰界面处的复合，而非预形成激子的输运。
• 环状发光持续时间超过一微秒，比中心区域的本征激子寿命长出多个数量级。
• 环半径随激发功率呈亚线性增长，且存在环形成的明确阈值功率。
• 环状发光光谱保持窄、对称且无位移，表明为低密度、热平衡的激子气体；而中心区域的发光随功率增加而展宽并红移。
• 时间分辨模拟显示，环状发光在微秒时间尺度上衰减，与实验观测一致；而中心区域的发光衰减时间约为50 ps。