Hydrodynamic nucleation of quantized vortex pairs in a polariton quantum fluid

Abstract

Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light-matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross-Pitaevskii equation.H ydrodynamic instabilities in classical fluids were studied in the pioneering experiments of Bénard in the 1910's. Convective Bénard-Rayleigh flows and Bénard-Von Kár-mán streets are now well known examples in nonlinear and chaos sciences 1 . In conventional fluids, the flow around an obstacle is characterized by the dimensionless Reynolds number Re = vR/ν, with v and ν the fluid velocity and dynamical viscosity, respectively, and R the diameter of the obstacle. When increasing the Reynolds number, laminar flow, stationary vortices, Bénard-Von Kármán streets of moving vortices and fully turbulent regimes are successively observed in the wake of the obstacle 1 .In quantum fluids, such as liquid helium or atomic BoseEinstein condensates, quantum turbulence has long been predicted to appear at the breakdown of superfluidity 2-8 . In superfluid systems, the Reynolds number cannot be defined owing to the absence of viscosity. However, quantum turbulence, in the form of quantized vortices, appears simultaneously with dissipation and drag on the obstacle once a critical velocity is exceeded. This critical velocity is predicted to be lower than the Landau criterion for superfluidity far from the obstacle, because of a local increase of the fluid velocity in the vicinity of the impenetrable obstacle 2,4,5 .Experimental evidence has been given for the appearance of a drag force or heat above some critical velocity in superfluid helium 5 and atomic Bose-Einstein condensates 9,10 . In stirred atomic gases, vortex lattices appear above a critical stirring frequency 11-13 , analogously to the rotating bucket experiments originally performed with superfluid helium 14 . Irregular vortex tangle patterns were also observed under an external oscillating perturbation, indicating the presence of turbulence in the atomic cloud 15 . Finally, vortex nucleation has been reported in the wake of a blue-detuned laser moving above a critical velocity through the condensate 16,17 . However, no experiment has yet allowed the imaging of the hydrodynamic nucleation mechanism with ...

Vortices in Polariton OPO Superfluids

“…In this way, in the region unstable for parametric scattering determined with the method described above, one can describe first the increase (switch-on) and later the decrease (switch-off) of the signal and idler populations as a function of the pump power. It is interesting to note that by doing that, i.e., by substituting (15) into (11), "satellite states" oscillating with energies ω s 2 = 2ω s − ω p = ω p − 2ω and ω i 2 = 2ω i − ω p = ω p + 2ω automatically appear. In fact, above OPO threshold, when signal and idler populations are not negligible, parametric scattering from the signal (idler) state into the pump and second-signal (second-idler) satellite state take place, i.e., 2s → p + s 2 (2i → p + i 2 ) and therefore 2ω s = ω p + ω s 2 (2ω i = ω p + ω i 2 ).…”

“…[11,12], the hydrodynamic nucleation of V-AV pairs is studied by making collide the polariton fluid with a large defect. In particular, polaritons are resonantly (coherently) injected with a pulsed laser beam, creating a population in the pump state only.…”

“…Their existence was first predicted in superfluids [1,2], and later in coherent waves [3]. Nowadays, quantised vortices have been the subject of extensive research across several areas of physics and have been observed in type-II superconductors, 4 He, ultracold atomic gases, non-linear optical media (for a review see, e.g., [4]) and very recently microcavity polaritons [5,6,7,8,9,10,11,12], the coherent strong mixing of a quantum well exciton with a cavity photon. adding a pulsed Gaussian probe to the OPO regime (the so called TOPO regime) in Sec.…”

Geometrically locked vortex lattices in semiconductor quantum fluids

“…Recently it has also proved possible to generate vortices by coherent resonant injection 17,18 together with flow against defects 19,20 . However, no observation of the predicted 21 spontaneous regular distribution of vortices in a lattice has yet been reported.…”

Dissociation dynamics of singly charged vortices into half-quantum vortex pairs

“…The recent experimental demonstration of polariton BEC in many different experimental configurations [7][8][9][10][11] , followed by the observation of polariton superfluidity 12,13 , has initiated the fascinating field of polariton quantum hydrodynamics. Particular attention has been devoted to the investigation of vortex excitations, both in the case of resonantly driven hydrodynamic experiments 14,15 and nonresonant 16,17 optical pumping.…”