We study, both theoretically and experimentally, the occurrence of topological defects in polariton superfluids in the optical parametric oscillator (OPO) regime. We explain in terms of local supercurrents the deterministic behaviour of both onset and dynamics of vortex-antivortex pairs generated by perturbing the system with a pulsed probe. Using a generalised Gross-Pitaevskii equation, including photonic disorder, pumping and decay, we elucidate the reason why topological defects form in couples and can be detected by direct visualizations in multi-shot OPO experiments.PACS numbers: 42.65. Yj, 71.36.+c Quantum vortices are topological defects occurring in macroscopically coherent systems. Their existence was first predicted in superfluids [1,2], and later in coherent waves [3]. Nowadays, quantum 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, nonlinear optics media (for a review see, e.g., [4,5]) and very recently microcavity polaritons [6][7][8][9][10][11][12][13]. The phase of a quantised vortex winds around its core from 0 to 2πm (with m integer), implying the vortex carries a quantised angular momentum, m. In contrast with the classical counterpart, quantum vortices with the same m are all identical, with a size (or healing length) determined by the system nonlinear properties.Recently, the study of quantized vortices imprinted in polariton condensates using pulsed laser fields has attracted noticeable interest both experimentally [9] and theoretically [14][15][16][17], providing a diagnostics for superfluid properties of such a non-equilibrium system. In particular, resonantly pumped polaritons in the OPO regime [18,19] have been recently shown to exhibit a new form of non-equilibrium superfluidity [9,20]. Here, polaritons continuously injected into the pump state, undergo coherent stimulated scattering into the signal and idler states. An additional pulsed probe can initiate a traveling decaying gain, which evolves freely from the probe constraints. By using a pulsed Laguerre-Gauss (LG) beam, vorticity has been shown to persist not only in absence of the rotating drive, but also longer than the gain induced by the probe, and therefore to be transferred to the OPO signal, demonstrating metastability of quantum vortices and persistence of currents [9,15].However, if the extension of the probe carrying a vortex with charge m = +1 is smaller than the size of the vortex-free OPO signal, continuity of the polariton wavefunction requires that necessarily an antivortex with charge m = −1 has to form at the edge of the probe (see Fig. 1). In this Letter, we demonstrate that 'unintended' antivortices do appear in the signal at the edge of the imprinting vortex probe and explain, both theoretically and via experiments, the origin of the deterministic behaviour of the antivortex onset and dynamics. In particular, we show where antivortices are more likely to appear in terms of the supercurrents of...

We study the linear response of a coherently driven polariton fluid in the pump-only configuration scattering against a point-like defect and evaluate analytically the drag force exerted by the fluid on the defect. When the system is excited near the bottom of the lower polariton dispersion, the sign of the interaction-renormalised pump detuning classifies the collective excitation spectra into three different categories (Ciuti and Carusotto 2005 Phys. Status Solidi b 242 2224): linear for zero, diffusive-like for positive and gapped for negative detuning. We show that both cases of zero and positive detuning share a qualitatively similar crossover of the drag force from the subsonic to the supersonic regime as a function of the fluid velocity, with a critical velocity given by the speed of sound found for the linear regime. In contrast, for gapped spectra, we find that the critical velocity exceeds the speed of sound. In all cases, the residual drag force in the subcritical regime depends on the polariton lifetime only. Also, well below the critical velocity, the drag force varies linearly with the polariton lifetime, in agreement with previous work (Cancellieri et al 2010 Phys. Rev. B 82 224512), where the drag was determined numerically for a finite-size defect.

Superfluidity, which is the ability of a liquid or gas to flow with zero viscosity, is one of the most remarkable implications of collective quantum coherence. In equilibrium systems such as liquid 4 He and ultracold atomic gases, superfluid behavior conjugates diverse yet related phenomena, such as a persistent metastable flow in multiply connected geometries and the existence of a critical velocity for frictionless flow when hitting a static defect. The link between these different aspects of superfluid behavior is far less clear in driven-dissipative systems displaying collective coherence, such as microcavity polaritons, which raises important questions about their concurrency. With a joint theoretical and experimental study, we show that the scenario is particularly rich for polaritons driven in a three-fluid collective coherent regime, i.e., a so-called optical parametric oscillator. On the one hand, the spontaneous macroscopic coherence following the phase locking of the signal and idler fluids has been shown to be responsible for their simultaneous quantized flow metastability. On the other hand, we show here that the pump, signal, and idler have distinct responses when hitting a static defect; while the signal displays modulations that are barely perceptible, the ones appearing in the pump and idler are determined by their mutual coupling due to nonlinear and parametric processes.

We theoretically study the driven-dissipative Harper-Hofstadter model on a 2D square lattice in the presence of a weak harmonic trap. Without pumping and loss, the eigenstates of this system can be understood, in certain limits, as momentum-space toroidal Landau levels, where the Berry curvature, a geometrical property of an energy band, acts like a momentum-space magnetic field. We show that key features of these eigenstates can be observed in the steady-state of the driven-dissipative system under a monochromatic coherent drive, and present a realistic proposal for an optical experiment using state-of-the-art coupled cavity arrays. We discuss how such spectroscopic measurements may be used to probe effects associated both with the off-diagonal elements of the matrix-valued Berry connection and with the synthetic magnetic gauge.

No abstract