David Vocke

Experimental characterization of nonlocal photon fluids

Quantum gases of atoms and exciton-polaritons are now well-established theoretical and experimental tools for fundamental studies of quantum many-body physics and suggest promising applications to quantum computing. Given their technological complexity, it is of paramount interest to devise other systems where such quantum many-body physics can be investigated at lesser technological expense. Here we examine a relatively well-known system of laser light propagating through thermo-optical defocusing media: based on a hydrodynamic description of light as a quantum fluid of interacting photons, we investigate such systems as a valid room-temperature alternative to atomic or exciton–polariton condensates for studies of many-body physics. First, we show that by using a technique traditionally used in oceanography it is possible to perform a direct measurement of the single-particle part of the dispersion relation of the elementary excitations on top of the photon fluid and to detect its global flow. Then, using a pump-and-probe setup, we investigate the dispersion of excitation modes of the fluid: for very long wavelengths, a sonic, dispersionless propagation is observed that we interpret as a signature of superfluid behavior.

From coherent shocklets to giant collective incoherent shock waves in nonlocal turbulent flows

Understanding turbulent flows arising from random dispersive waves that interact strongly through nonlinearities is a challenging issue in physics. Here we report the observation of a characteristic transition: strengthening the nonlocal character of the nonlinear response drives the system from a fully turbulent regime, featuring a sea of coherent small-scale dispersive shock waves (shocklets) towards the unexpected emergence of a giant collective incoherent shock wave. The front of such global incoherent shock carries most of the stochastic fluctuations and is responsible for a peculiar folding of the local spectrum. Nonlinear optics experiments performed in a solution of graphene nano-flakes clearly highlight this remarkable transition. Our observations shed new light on the role of long-range interactions in strongly nonlinear wave systems operating far from thermodynamic equilibrium, which reveals analogies with, for example, gravitational systems, and establishes a new scenario that can be common to many turbulent flows in photonic quantum fluids, hydrodynamics and Bose–Einstein condensates.

Optical analogues of the Newton–Schrödinger equation and boson star evolution

Many gravitational phenomena that lie at the core of our understanding of the Universe have not yet been directly observed. An example in this sense is the boson star that has been proposed as an alternative to some compact objects currently interpreted as being black holes. In the weak field limit, these stars are governed by the Newton–Schrodinger equation. Here we present an optical system that, under appropriate conditions, identically reproduces such equation in two dimensions. A rotating boson star is experimentally and numerically modelled by an optical beam propagating through a medium with a positive thermal nonlinearity and is shown to oscillate in time while also stable up to relatively high densities. For higher densities, instabilities lead to an apparent breakup of the star, yet coherence across the whole structure is maintained. These results show that optical analogues can be used to shed new light on inaccessible gravitational objects.

Rotating black hole geometries in a two-dimensional photon superfluid

Analogue gravity studies the physics of curved spacetime in laboratory experiments, where the propagation of elementary excitations in inhomogeneous flows is mapped to those of scalar fields in a curved spacetime metric. While most analogue gravity experiments are performed in 1+1 dimensions (one spatial plus time) and thus can only mimic only 1+1D spacetime, we present a 2+1D photon (room temperature) superfluid where the geometry of a rotating acoustic black hole can be realized in 2+1D dimensions. By measuring the local flow velocity and speed of waves in the superfluid, we identify a 2D region surrounded by an ergo sphere and a spatially separated event horizon. This provides the first direct experimental evidence of an ergosphere and horizon in any system, and the possibility in the future to study the analogue of Penrose superradiance from rotating black holes with quantised angular momentum and modified dispersion relations.

Shock-induced complex phase-space dynamics of strongly turbulent flows

Shock waves have been thoroughly investigated during the last century in many different branches of physics. In conservative (Hamiltonian) systems the shock singularity is regularized by weak wave dispersion, thus leading to the formation of a rapidly and regular oscillating structure, usually termed in the literature dispersive shock wave (DSW), see e.g. [1]. Here, we show that this fundamental singular process of DSW formation can break down in a system of incoherent nonlinear waves. We consider the strong turbulent regime of a system of nonlocal nonlinear optical waves. We report theoretically and experimentally a characteristic transition: Strengthening the nonlocal character of the nonlinear response drives the system from a fully turbulent regime, featuring a sea of coherent small-scale DSWs (shocklets) towards the unexpected emergence of a giant collective incoherent shock wave [2]. Then contrary to conventional shocks that are inherently coherent deterministic entities, here the giant shock is a collective phenomenon of the incoherent field as a whole.

Two-dimensional acoustic horizon and ergosphere in a nonlocal photon superfluid

Analogue gravity studies the physics of curved spacetime via an analogy with laboratory physics, opening a new window to test the concepts of general relativity experimentally. Sound waves in an inhomogeneous fluid flow behave as a scalar field in a curved spacetime metric. In this context, an acoustic horizon is understood as a surface or area in space where the orthogonal component of the flow speed across that surface equals the speed of sound resulting in a blocking of counter-propagating waves. Recently, quantum fluids of light have attracted particular interest in creating hydrodynamic analogues that are able to re-construct 2D spacetimes of black holes [1]. Photon fluids in a propagating geometry, where the fluid is established by the effective photon-photon interaction in a defocusing nonlinear medium, provide experimentally easy access to superfluidity that is vital for such studies [2]. Here we show that the superfluid character of the photon fluid allows us to build 2D spacetime geometries. By controlling the intensity and the topology of the spatial phase of the beam, we are able to identify a 2D black hole horizon and ergosphere in an analogue system for the first time. Our photon fluid is established in the transverse plane of a paraxially propagating CW laser beam, that is launched through a methanol/graphene solution with a thermal optical nonlinearity. The flow and speed of sound is a function of the spatial phase gradient and intensity of the laser, so controlling these parameters allows easy implementation of 2D flow geometries. With this, we realise the 2D spacetime of a rotating black hole and experimentally map the spatial structure of the local speed of sound of phonon-like excitations and the total flow in the photon fluid to identify an acoustic horizon and ergosphere (Fig. 1). We then add a weak probe beam to create small amplitude excitations in the black hole flow and experimentally observe scattering of these waves from a rotating spacetime, where the scattered part carries away angular momentum. Numerical simulations using the Nonlinear Schrodinger equation allow us to identify the scattering terms responsible and measure the probes orbital angular momentum (OAM) spectrum. We see amplification of phonons that co propagate with the rotation (far right of Fig. 1), hence gaining energy from the rotating black hole, an effect that shares similarities to Penrose superradiance predicted for astrophysical black holes [3]. Summarising, we present experimental evidence of a two-dimensional horizon and ergosphere of an analogue, rotating black hole and study the scattering and instabilities of small amplitude waves in such a spacetime.

Two dimensional acoustic horizon and ergosphere in a nonlocal photon superfluid

We present experimental evidence of a two-dimensional black hole horizon and ergosphere for the first time in an analogue system using a nonlocal photon fluid based on a thermal nonlinearity.

Giant collective incoherent shock waves in strongly nonlinear turbulent flows

Contrary to conventional coherent shocks, we show theoretically and experimentally that nonlocal turbulent flows lead to the emergence of large-scale incoherent shock waves, which constitute a collective phenomenon of the incoherent field as a whole.

The role of geometry in nonlocal superfluids

By modifying the geometry of the optical beam, we control the effective nonlocal interaction length of a nonlocal photon fluid. This allows us to access the superfluid flow regime, which would otherwise be unobservable.

Emergent geometries and nonlinear-wave dynamics in photon fluids

Nonlinear waves in defocusing media are investigated in the framework of the hydrodynamic description of light as a photon fluid. The observations are interpreted in terms of an emergent curved spacetime generated by the waves themselves, which fully determines their dynamics. The spacetime geometry emerges naturally as a result of the nonlinear interaction between the waves and the self-induced background flow. In particular, as observed in real fluids, different points of the wave profile propagate at different velocities leading to the self-steepening of the wave front and to the formation of a shock. This phenomenon can be associated to a curvature singularity of the emergent metric. Our analysis offers an alternative insight into the problem of shock formation and provides a demonstration of an analogue gravity model that goes beyond the kinematic level.