Unstable and stable regimes of polariton condensation

Modulational instabilities play a key role in a wide range of nonlinear optical phenomena, leading, e.g., to the formation of spatial and temporal solitons, rogue waves, and chaotic dynamics. Here, we experimentally demonstrate the existence of a modulational instability in condensates of cavity polaritons, quasi-particles arising from the strong coupling of cavity photons with quantum well excitons. For this purpose, we investigate the spatiotemporal coherence properties of polariton condensates in GaAs-based microcavities under continuous-wave pumping. The chaotic behavior of the instability results in a strongly reduced spatial and temporal coherence and a significantly inhomogeneous density. Additionally, we show how the instability can be tamed by introducing a periodic potential so that condensation occurs in negative mass states, leading to largely improved coherence and homogeneity. These results pave the way to the exploration of long-range order in dissipative quantum fluids of light within a controlled platform.