Analogue gravity in nonlocal fluids of light
Vocke, David Emanuel Frank
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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.