Polariton thermalization is a key process in achieving light–matter Bose–Einstein condensation, spanning from solid-state semiconductor microcavities at cryogenic temperatures to surface plasmon nanocavities with molecules at room temperature. Originated from the matter component of polariton states, the microscopic mechanisms of thermalization are closely tied to specific material properties. In this work, we investigate polariton thermalization in strongly-coupled molecular systems. We develop a microscopic theory addressing polariton thermalization through electron-phonon interactions (known as exciton-vibration coupling) with low-energy molecular vibrations. This theory presents a simple analytical method to calculate the temperature-dependent polariton thermalization rate, utilizing experimentally accessible spectral properties of bare molecules, such as the Stokes shift and temperature-dependent linewidth of photoluminescence, in conjunction with well-known parameters of optical cavities. Our findings demonstrate qualitative agreement with recent experimental reports of nonequilibrium polariton condensation in both ground and excited states, and explain the thermalization bottleneck effect observed at low temperatures. This study showcases the significance of vibrational degrees of freedom in polariton condensation and offers practical guidance for future experiments, including the selection of suitable material systems and cavity designs.

中文翻译：

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强耦合分子系统中极化激元的热化速率

极化子热化是实现光-物质玻色-爱因斯坦凝聚的关键过程，从低温下的固态半导体微腔到室温下分子的表面等离子体纳米腔。热化的微观机制源于极化子态的物质成分，与特定的材料特性密切相关。在这项工作中，我们研究了强耦合分子系统中的极化子热化。我们开发了一种微观理论，通过低能分子振动的电子-声子相互作用（称为激子-振动耦合）来解决极化子热化问题。该理论提出了一种简单的分析方法来计算与温度相关的极化子热化率，利用裸分子的实验可获取的光谱特性，例如斯托克斯位移和光致发光的与温度相关的线宽，以及众所周知的光学腔参数。我们的研究结果表明与最近基态和激发态非平衡极化子凝聚的实验报告定性一致，并解释了在低温下观察到的热化瓶颈效应。这项研究展示了极化子凝聚中振动自由度的重要性，并为未来的实验提供了实用指导，包括选择合适的材料系统和腔体设计。