# Superluminal X-waves in a polariton quantum fluid

## TL;DR

This paper reports the first experimental observation of spontaneous two-dimensional exciton-polariton X-waves, which maintain their shape without spreading due to their unique dispersion and nonlinearity, enabling superluminal propagation at low densities.

## Contribution

The study demonstrates the experimental generation and control of polariton X-waves, highlighting their shape-preserving properties and superluminal propagation in a linear and nonlinear regime.

## Key findings

- X-waves are experimentally generated in polariton fluids.
- X-waves propagate without spreading at very low densities.
- Superluminal peak propagation is tunable and matches simulations.

## Abstract

In this work we experimentally demonstrate for the first time spontaneous generation of two-dimensional exciton-polariton X-waves. X-waves belong to the family of localized packets, which are capable of sustaining their shape with no spreading even in the linear regime. This allows to keep the packet shape and size for very low densities and very long times compared, for instance, to soliton waves, which always necessitate a nonlinearity to compensate the diffusion. Here we exploit the polariton nonlinearity and unique structured dispersion, comprising both positive- and negative-mass curvatures, to trigger an asymmetric four wave mixing in the momentum space. This ultimately enables self-formation of a spatial X-wave front. By means of ultrafast imaging experiments we observe the early reshaping of the initial Gaussian packet into the X-pulse and its propagation even for vanishing small densities. This allows us to outline the crucial effects and parameters driving the phenomena and to tune the degree of peak superluminal propagation, which we found to be in a good agreement with numerical simulations.