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Register a new userAnnular Vortex Chain in a Resonantly Pumped Polariton Superfluid
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We report the formation of a ring-shaped array of vortices after injection of angular momentum in a polariton superfluid. The angular momentum is injected by a $ell= 8$ Laguerre-Gauss beam, whereas the global rotation of the fluid is hindered by a narrow Gaussian beam placed at its center. In the linear regime a spiral interference pattern containing phase defects is visible. In the nonlinear (superfluid) regime, the interference disappears and the vortices nucleate as a consequence of the angular momentum quantization. The radial position of the vortices evolves freely in the region between the two pumps as a function of the density. Hydrodynamic instabilities resulting in the spontaneous nucleation of vortex-antivortex pairs when the system size is sufficiently large confirm that the vortices are not constrained by interference when nonlinearities dominate the system.
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We observe for the first time two-photon excited condensation of exciton-polaritons. The angle-resolved photoluminescence (PL) from the Lower Polariton (LP) ground state in our planar GaAs-based microcavity structure exhibits a clear intensity threshold as a function of increased two-photon excitation power, coinciding with an interaction-induced blueshift and a narrowing of spectral linewidth, characteristic of the transition from a thermal distribution of lower polaritons to polariton condensation. Two-Photon Absorption (TPA) is evidenced in the quadratic dependence of the input-output curves below and above the threshold region. Second Harmonic Generation (SHG) is ruled out by both this threshold behavior and by scanning the pump photon energy and observing a lack of dependence of the LP emission peak energy. Our results pave the way towards realization of a polariton-based stimulated THz radiation source, stemming from the dipole-allowed transition from the Quantum Well (QW) 2p dark exciton state to the 1s-exciton-based LP ground state, as theoretically predicted in [A. V. Kavokin et al., Phys. Rev. Lett. 108, 197401 (2012)].
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 in three different categories [C. Ciuti and I. Carusotto, physica status solidi (b) 242, 2224 (2005)]: 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 [E. Cancellieri et al., Phys. Rev. B 82, 224512 (2010)], where the drag was determined numerically for a finite-size defect.
We present a theoretical study of the hydrodynamic properties of a quantum gas of exciton-polaritons in a semiconductor microcavity under a resonant laser excitation. The effect of a spatially extended defect on the superfluid flow is investigated as a function of the flow speed. The processes that are responsible for the nucleation of vortices and solitons in the wake of the defect are characterized, as well as the regimes where the superfluid flow remains unperturbed. Specific features due to the non-equilibrium nature of the polariton fluid are put in evidence.
The imbalanced superfluid state of spin-1/2 fermions with s-wave pairing is numerically studied by solving the Bogoliubov-de-Gennes equation at zero temperature in an annular disk geometry with narrow radial width. Two distinct types of systems are considered. The first case may be relevant to heavy fermion superconductors, where magnetic field causes spin imbalance via Zeeman interaction and the system is studied in a grand canonical ensemble. As the magnetic field increases, the system is transformed from the uniform superfluid state to the Fulde-Ferrell-Larkin-Ovchinnikov state, and finally to the spin polarized normal state. The second case may be relevant to cold fermionic systems, where the numbers of fermions of each species are fixed as in a canonical ensemble. In this case, the groundstate depends on the pairing strength. For weak pairing, the order parameter exhibits a periodic domain wall lattice pattern with a localized spin distribution at low spin imbalance, and a sinusoidally modulated pattern with extended spin distribution at high spin imbalance. For strong pairing, the phase separation between superfluid state and polarized normal state is found to be more preferable, while the increase of spin imbalance simply changes the ratio between them.
We study the properties of a binary microcavity polariton superfluid coherently injected by two lasers. The crossover from the supersonic to subsonic regime, where motion is frictionless, is described by evaluating the Bogoliubov spectra. We show that according to the Landau criteria, the coupling between the two components precludes the existence of superfluidity just for one component but not for the other. By analysing the drag force exerted on a defect, we give a recipe to experimentally address the crossover from the supersonic to the subsonic regime.
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