Analogue gravity in nonlocal fluids of light

Abstract

Analogue gravity designates the study of curved spacetime in a laboratory environment and allows to test concepts of General Relativity. This analogy is established via a conformal identity between the flow of curved spacetime and inhomogeneous flows in hydrodynamics, which predicts that small waves on a fluid behave exactly as scalar fields in a curved spacetime metric. Atomic quantum fluids such as Bose-Einstein Condensates (BEC) are a widespread workbench for studying artificial black holes and many-body physics but face considerably large experimental challenges. In recent years, quantum fluids of light became a promising alternative at less technical expense, where the many-body dynamics in a laser beam are established via photon-photon interactions mediated through an optical nonlinearity. Whereas recent works considered strongly confined laser fields in microcavities, this work presents a photon fluid in a propagating geometry, i.e. a paraxially propagating laser beam in a bulk nonlinear medium. In this scenario, the propagating direction maps onto a time coordinate and the photon fluid is established in the transverse beam profile. The thermal nonlinearity is excited through heating of the absorbed laser power that introduces a nonlocal response of the medium and adds another level of complexity. It is experimentally shown that the dynamics of small amplitude excitations are governed by the Bogoliubov dispersion relation and allows to observe superfluidity at sufficiently large wavelengths. This is confirmed by the onset of persistent currents and the nucleation of quantized vortices in sub- and supercritical flows around an extended obstacle, which is a direct observation of superfluidity in a room-temperature system. The superfluid regime is a requirement for building analogue spacetime metrics and is thus of paramount importance. The spacetime of a rotating black and white whole was then created by shaping the topology of the spatial phase using diffractive phase masks. The experimental measurements of the inhomogeneous flows revealed, for the first time conclusive evidence of a (2+1) dimensional acoustic horizon and ergosphere. Such a system promises to study Penrose superradiance, where first experimental and numerical results for its observation are presented. Finally, nonlinear wave dynamics such as self-steepening and shock formation are studied where the dynamics can be interpreted in terms of a self-induced spacetime. Furthermore, the dynamics of a sea of incoherent waves is studied with respect to the long-range interactions provided by the nonlocality, where a novel transition from individual dispersive shock waves towards a collective giant shock wave is observed.