Dispersion relation of the collective excitations in a resonantly driven polariton fluid

TL;DR

This study measures the dispersion relation of collective excitations in a resonantly driven polariton fluid, revealing the significant influence of a long-lived exciton reservoir on the fluid's properties and dynamics.

Contribution

It provides the first direct measurement of the dispersion relation in such systems and demonstrates the reservoir's crucial role in polariton hydrodynamics.

Findings

Measured dispersion relation via Brillouin scattering.

Identified the reservoir's impact on the speed of sound.

Quantitatively explained the low speed of sound with reservoir effects.

Abstract

Exciton-polaritons in semiconductor microcavities constitute the archetypal realization of a quantum fluid of light. Under coherent optical drive, remarkable effects such as superfluidity, dark solitons or the nucleation of hydrodynamic vortices have been observed. These phenomena can be all understood as a specific manifestation of collective excitations forming on top of the polariton condensate. In this work, we performed a Brillouin scattering experiment to measure their dispersion relation $\omega(\mathbf{k})$ directly. The result, such as a speed of sound which is apparently twice too low, cannot be explained upon considering the polariton condensate alone. In a combined theoretical and experimental analysis, we demonstrate that the presence of a reservoir of long-lived excitons interacting with polaritons has a dramatic influence on the nature and characteristic of the quantum fluid, and that it explains our measurement quantitatively. This work clarifies the role of such a reservoir in the different polariton hydrodynamics phenomena occurring under resonant optical drive. It also provides an unambiguous tool to determine the condensate-to-reservoir fraction in the quantum fluid, and sets an accurate framework to approach novel ideas for polariton-based quantum-optical applications.