Spectrum of collective excitations of a quantum fluid of polaritons

We use a recently developed high-resolution coherent probe spectroscopy method to investigate the dispersion of collective excitations polaritonic quantum fluid. measure relation with high energy and wavenumber resolution, which allows us determine speed sound in fluid evidence contribution an excitonic reservoir. report on generation at negative energies, ghost branch curve. Precursors dynamical instabilities are also identified. Our methods open way precise study hydrodynamics fluids light.