Quantum fluids of light are a photonic counterpart to atomic Bose gases and are attracting increasing interest for probing many-body physics quantum phenomena such as superfluidity. Two different configurations are commonly used: the confined geometry where a nonlinear material is fixed inside an optical cavity and the propagating geometry where the propagation direction plays the role of an effective time for the system. The observation of the dispersion relation for elementary excitations in a photon fluid has proved to be a difficult task in both configurations with few experimental realizations. Here, we propose and implement a general method for measuring the excitations spectrum in a fluid of light, based on a group velocity measurement. We observe a Bogoliubov-like dispersion with a speed of sound scaling as the square root of the fluid density. This Letter demonstrates that a nonlinear system based on an atomic vapor pumped near resonance is a versatile and highly tunable platform to study quantum fluids of light.

We implement Bragg-like spectroscopy in a paraxial fluid of light by imprinting analogues of short Bragg pulses on the photon fluid using wavefront shaping with a spatial light modulator. We report a measurement of the static structure factor, S(k), and we find a quantitative agreement with the prediction of the Feynman relation revealing indirectly the presence of pair-correlated particles in the fluid. Finally, we improve the resolution over previous methods and obtain the dispersion relation including a linear phononic regime for weakly interacting photons and low sound velocity.

Characterizing elementary excitations in quantum fluids is essential to study their collective effects. We present an original angle-resolved coherent probe spectroscopy technique to study the dispersion of these excitation modes in a fluid of polaritons under resonant pumping. Thanks to the unprecedented spectral and spatial resolution, we observe directly the low-energy phononic behavior and detect the negative-energy modes, i.e., the ghost branch, of the dispersion relation. In addition, we reveal narrow spectral features precursory of dynamical instabilities due to the intrinsic out-of-equilibrium nature of the system. This technique provides the missing tool for the quantitative study of quantum hydrodynamics in polariton fluids.

The general theory of dark solitons relies on repulsive interactions and, therefore, predicts the impossibility to form dark-soliton bound states. One important exception to this prediction is the observation of bound solitons in nonlocal nonlinear media. Here, we report that exciton-polariton superfluids can also sustain dark-soliton molecules, although the interactions are fully local. With a novel all-optical technique, we create two dark solitons that bind together to form an unconventional dark-soliton molecule. We demonstrate that the stability of this structure and the separation distance between two dark solitons is tightly connected to the driven-dissipative nature of the polariton fluid.