Drag in a resonantly driven polariton fluid

TL;DR

This paper analyzes how a coherently driven polariton fluid responds to a defect, revealing how the drag force varies with fluid velocity, detuning, and polariton lifetime, and identifying different excitation regimes.

Contribution

It provides an analytical study of the drag force in polariton fluids, classifying excitation spectra and revealing the dependence of critical velocity on the spectrum type.

Findings

Drag force exhibits a subsonic to supersonic crossover at the speed of sound for linear and diffusive spectra.

For gapped spectra, the critical velocity exceeds the speed of sound.

Residual drag in the subcritical regime depends on polariton lifetime and varies linearly with it.

Abstract

We study the linear response of a coherently driven polariton fluid in the pump-only configuration scattering against a point-like defect and evaluate analytically the drag force exerted by the fluid on the defect. When the system is excited near the bottom of the lower polariton dispersion, the sign of the interaction-renormalised pump detuning classifies the collective excitation spectra in three different categories [C. Ciuti and I. Carusotto, physica status solidi (b) 242, 2224 (2005)]: linear for zero, diffusive-like for positive, and gapped for negative detuning. We show that both cases of zero and positive detuning share a qualitatively similar crossover of the drag force from the subsonic to the supersonic regime as a function of the fluid velocity, with a critical velocity given by the speed of sound found for the linear regime. In contrast, for gapped spectra, we find that the critical velocity exceeds the speed of sound. In all cases, the residual drag force in the subcritical regime depends on the polariton lifetime only. Also, well below the critical velocity, the drag force varies linearly with the polariton lifetime, in agreement with previous work [E. Cancellieri et al., Phys. Rev. B 82, 224512 (2010)], where the drag was determined numerically for a finite-size defect.