Directional Goldstone waves in polariton condensates close to equilibrium

Dario Ballarini,Giuseppe Gigli,Marzena H Szymańska,Loren N Pfeiffer,Richard Juggins,Davide Caputo,Daniele Sanvitto,Lorenzo Dominici,Milena De Giorgi,Kenneth West,Galbadrakh Dagvadorj

doi:10.1038/s41467-019-13733-x

Abstract

Quantum fluids of light are realized in semiconductor microcavities using exciton-polaritons, solid-state quasi-particles with a light mass and sizeable interactions. Here, we use the microscopic analogue of oceanographic techniques to measure the excitation spectrum of a thermalised polariton condensate. Increasing the fluid density, we demonstrate the transition from a free-particle parabolic dispersion to a linear, sound-like Goldstone mode characteristic of superfluids at equilibrium. Notably, we reveal the effect of an asymmetric pumping by showing that collective excitations are created with a definite direction with respect to the condensate. Furthermore, we measure the critical sound speed for polariton superfluids close to equilibrium.

Highlights

Quantum fluids of light are realized in semiconductor microcavities using exciton-polaritons, solid-state quasi-particles with a light mass and sizeable interactions
The fundamental dynamics of the collective excitations of polariton quantum fluids are often hidden by the microscopic details of the disorder environment and by the effect of pumping and dissipation
The excitation spectrum of polariton condensates is modified by drive and decay with respect to the equilibrium case of cold atoms, and is diffusive rather than linear at small momenta[7,8]

Summary

Directional Goldstone waves in polariton condensates close to equilibrium

Dario Ballarini 1*, Davide Caputo[1,2], Galbadrakh Dagvadorj[3,4], Richard Juggins[3], Milena De Giorgi[1], Lorenzo Dominici[1], Kenneth West[5], Loren N. We employ a high-quality semiconductor microcavity with a reduced density of defects and long polariton lifetime to form a condensate close to equilibrium and to measure the spectrum of its collective excitations by interferometric measurements of temporal and spatial coherence This represents the optical analogue of the oceanographic technique used to obtain the dispersion of surface waves (Supplementary Note 1). Collective excitations form with a preferential direction, much like surface waves in the ocean are oriented along the wind blowing in a specific direction The origin of this peculiar effect in polariton condensates is due to the spatial displacement of the exciton reservoir from the polariton condensate, which results in an asymmetrically populated Goldstone dispersion

Results

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Additional information