Dark and thermal reservoir contributions to polariton sound velocity

TL;DR

This paper theoretically investigates how dark and thermal reservoir populations influence the sound velocity in polariton condensates, revealing that dark excitons significantly lower the sound speed and alter its dependence on system parameters.

Contribution

It introduces a model to quantify dark exciton effects on polariton sound velocity and proposes a method to experimentally determine dark exciton density.

Findings

Dark excitons lower the polariton sound velocity.

The model links sound velocity deviations to dark exciton density.

Different pumping schemes affect the condensate-reservoir dynamics.

Abstract

Exciton-polaritons in an optical microcavity can form a macroscopically coherent state despite being an inherently driven-dissipative system. In comparison with equilibrium bosonic fluids, polaritonic condensates possess multiple peculiarities that make them behave differently from well-known textbook examples. One such peculiarity is the presence of dark excitons which are created by the pump together with optically-active particles. They can considerably affect the spectrum of elementary excitations of the condensate and hence change its superfluid properties. Here, we theoretically analyze the influence of the bright and dark ``reservoir'' populations on the sound velocity $c_s$ of incoherently-driven polaritons. Both pulsed and continuous-wave pumping schemes characterized by essentially different condensate-to-reservoir ratio are considered. We show that the dark exciton contribution leads to considerable lowering of $c_s$ and to its deviation from the square-root-like behavior on the system's chemical potential (measurable condensate blueshift). Importantly, our model allows to unambiguously define the density of dark excitons in the system by experimentally tracking $c_s$ against the condensate blueshift and fitting the dependence at a given temperature.