Mechanistic principles of exciton-polariton relaxation

TL;DR

This paper uncovers the microscopic mechanisms of exciton-polariton relaxation, revealing a two-step phonon-induced process and how material thickness influences relaxation rates through phonon-fluctuation synchronization.

Contribution

It provides a detailed mechanistic understanding of exciton-polariton relaxation, including the effects of material thickness and phonon-fluctuation synchronization, supported by simulations and analytical analysis.

Findings

Phonon-induced relaxation involves a two-step process: inter-band transition then intraband scattering.

Finite thickness materials suppress intraband phonon scattering due to phonon-fluctuation synchronization.

Derived analytical expressions relate material thickness to relaxation rate constants.

Abstract

Exciton-polaritons are light-matter hybrid quasi-particles that have emerged as a flexible platform for developing quantum technologies and engineering material properties. However, the fundamental mechanistic principles that govern their dynamics and relaxation remain elusive. In this work, we provide the microscopic mechanistic understanding of the exciton-polariton relaxation process that follows from an excitation in the upper polariton. Using both mixed quantum-classical simulations and analytical analysis, we reveal that phonon-induced upper-to-lower polariton relaxation proceeds via two steps: the first step is a vertical inter-band transition from the upper to the lower polariton, which is followed by a second step that is a phonon-induced Fr\"ohlich scattering within the lower polariton. We find that in materials of finite thickness (which include filled cavities), phonon-induced polaritonic intraband Fr\"ohlich scattering is significantly suppressed. We show that the microscopic origin of this suppression is phonon-fluctuations synchronization (or self-averaging) due to the polaritonic spatial delocalization in the quantization direction. Finally, we show that the same phonon fluctuation-synchronization effect plays a central role across polaritonic relaxation pathways, and we derive simple analytical expressions that relate a material's finite thickness to the corresponding relaxation rate constants.