Spin and Density Modes in a Binary Fluid of Light

We report 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 realized by propagating the two circular polarization components of a laser through a nonlinear hot atomic vapor in the paraxial limit. By controlling the intensity and phase of both polarizations, we 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 sound velocities, corresponding to the spin and density modes. At high photon density, we observe a crossing of these branches, resulting from higher order nonlinear terms, linked to the saturation of the medium's nonlinearity.