Spin and Density Modes in a Binary Fluid of Light

TL;DR

This paper reports the experimental observation of spin and density modes in a binary superfluid of light, revealing their dispersion relations and the effects of photon-photon interactions on their crossing behavior.

Contribution

It demonstrates the control and measurement of spin and density modes in a light-based binary superfluid, including the observation of branch crossing due to nonlinear interactions.

Findings

Identification of distinct spin and density mode branches

Observation of branch crossing at high photon densities

Control over the ratio of sound velocities in the mixture

Abstract

We present the experimental observation of spin and density modes in a binary mixture of superfluids of light. A miscible Bose-Bose mixture with repulsive interactions is obtained by propagating, in the paraxial limit, the two circular polarization components of a laser through a non-linear hot atomic vapor. Controlling the intensity and phase for both polarizations allows us to selectively excite the fundamental modes of the mixture. Using a Bragg-like spectroscopy technique, we measure the dispersion relation and identify two distinct branches with different speeds of sound corresponding to the spin and density modes. At large photon density, we observe a crossing of these branches, which is due to an effective photon-photon interaction beyond the two-body regime and related to the saturation of the medium nonlinearity. This novel degree of freedom allows for precise control over the ratio of spin and density sound velocities and provide new insights into the control of binary mixtures collective dynamics.