Superluminal X-waves in a polariton quantum fluid

In this work, we experimentally demonstrate for the first time the spontaneous generation of two-dimensional exciton-polariton X-waves. X-waves belong to the family of localized packets that can sustain their shape without spreading, even in the linear regime. This allows the wavepacket to maintain its shape and size for very low densities and very long times compared to soliton waves, which always necessitate a nonlinearity to compensate the diffusion. Here, we exploit the polariton nonlinearity and uniquely structured dispersion, comprising both positive- and negative-mass curvatures, to trigger an asymmetric four-wave mixing in momentum space. This ultimately enables the self-formation of a spatial X-wave front. Using ultrafast imaging experiments, we observe the early reshaping of the initial Gaussian packet into the X-pulse and its propagation, even for vanishingly small densities. This allows us to outline the crucial effects and parameters that drive the phenomena and to tune the degree of superluminal propagation, which we found to be in close agreement with numerical simulations.