Second Thresholds in BEC-BCS-Laser Crossover of Exciton-Polariton Systems

TL;DR

This paper theoretically investigates the second thresholds in exciton-polariton systems, revealing two distinct types related to phase crossover and lasing, distinguished by band renormalization effects and gain spectra.

Contribution

It identifies and characterizes two different types of second thresholds in the BEC-BCS-laser crossover of exciton-polariton systems, highlighting the role of band renormalization.

Findings

Two types of second thresholds: crossover within quasi-equilibrium and into non-equilibrium.

Band renormalization causes gaps indicating bound electron-hole pairs.

Gain spectra can distinguish the two threshold types.

Abstract

The mechanism of second thresholds observed in several experiments is theoretically revealed by studying the BEC-BCS-laser crossover in exciton-polariton systems. We found that there are two different types for the second thresholds; one is a crossover within quasi-equilibrium phases and the other is into non-equilibrium (lasing). In both cases, the light-induced band renormalization causes gaps in the conduction and valence bands, which indicates the exsistence of bound electron-hole pairs in contrast to earlier expectations. We also show that these two types can be distinguished by the gain spectra.