arXiv subjects

D. Ballarini

Publications and source records attributed to D. Ballarini.

27 records · Page 2Linked to original sources

arXiv2015

Vortex and half-vortex dynamics in a spinor quantum fluid of interacting polaritons

Spinorial or multi-component Bose-Einstein condensates may sustain fractional quanta of circulation, vorticant topological excitations with half integer windings of phase and polarization. Matter-light quantum fluids, such as microcavity polaritons, represent a unique test bed for realising strongly interacting and out-of-equilibrium condensates. The direct access to the phase of their wavefunction enables us to pursue the quest of whether half vortices ---rather than full integer vortices--- are the fundamental topological excitations of a spinor polariton fluid. Here, we are able to directly generate by resonant pulsed excitations, a polariton fluid carrying either the half or full vortex states as initial condition, and to follow their coherent evolution using ultrafast holography. Surprisingly we observe a rich phenomenology that shows a stable evolution of a phase singularity in a single component as well as in the full vortex state, spiraling, splitting and branching of the initial cores under different regimes and the proliferation of many vortex anti-vortex pairs in self generated circular ripples. This allows us to devise the interplay of nonlinearity and sample disorder in shaping the fluid and driving the phase singularities dynamics

cond-mat.quant-gas↗

arXiv2015

Real-space collapse of a polariton condensate

Polaritons in microcavities are versatile quasi-2D bosonic particles with a high degree of coherence and strong nonlinearities, thanks to their hybrid light-matter character. In their condensed form, they display striking quantum hydrodynamic features analogous to atomic Bose-Einstein condensates, such as long-range order coherence, superfluidity and quantized vorticity. Their variegated dispersive and dissipative properties, however, set significant differences from their atomic counterpart. In this work, we report the unique phenomenology that is observed when a pulse of light impacts the polariton vacuum: the condensate that is instantaneously formed does not splash in real space but instead coheres into an enigmatic structure, featuring concentric rings and, most notably, a sharp and bright peak at the center. Using a state-of-the-art ultrafast imaging with 50 fs time steps, we are able to track the dynamics of the polariton mean-field wavefunction in both real and reciprocal space. The observation of the real-space collapse of the condensate into an extremely localized---resolution limited---peak is at odd with the repulsive interactions of polaritons and their positive effective mass. An unconventional mechanism is therefore at play to account for our observations. Our modeling suggests that self-trapping due to a local heating of the crystal lattice---that can be described as a collective polaron formed by a polariton condensate---could be involved. These observations hint at the fascinating fluid dynamics of polaritons in conditions of extreme intensities and ultrafast times.

cond-mat.quant-gas↗

arXiv2015

On multicomponent polariton superfluidity in the optical parametric oscillator regime

Superfluidity, 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 like liquid 4He and ultracold atomic gases, superfluid behaviour conjugates diverse yet related phenomena, such as persistency of 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 behaviour 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 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 pump, signal and idler have distinct responses when hitting a static defect; while the signal displays hardly appreciable modulations, the ones appearing in pump and idler are determined by their mutual coupling due to nonlinear and parametric processes.

cond-mat.mes-hall↗

arXiv2014

Spanning the full Poincaré sphere with polariton Rabi oscillations

We propose theoretically and demonstrate experimentally a generation of light pulses whose polarization varies temporally to cover selected areas of the Poincaré sphere with tunable swirling speed and total duration (1 ps and 10 ps respectively in our implementation). The effect relies on the Rabi oscillations of two polarized fields in the strong coupling regime, excited by two counter-polarized and delayed pulses. The interferences of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle on the sphere of controllable radius to a single point at long times. The positioning of the initial and final states allows to engineer the type of polarization spanning, including a full sweeping of the Poincaré sphere. The universality and simplicity of the scheme should allow for the deployment of time varying polarization fields at a technologically exploitable level.

cond-mat.quant-gas↗

arXiv2014

Ultrafast control of Rabi oscillations in a polariton condensate

We report the experimental observation and control of space and time-resolved light-matter Rabi oscillations in a microcavity. Our setup precision and the system coherence are so high that coherent control can be implemented with amplification or switching off of the oscillations and even erasing of the polariton density by optical pulses. The data is reproduced by a fundamental quantum optical model with excellent accuracy, providing new insights on the key components that rule the polariton dynamics.

cond-mat.quant-gas↗

arXiv2013

Control and ultrafast dynamics of a two-fluid polariton switch

We investigate the cross-interactions in a two-component polariton quantum fluid coherently driven by two independent pumping lasers tuned at different energies and momenta. We show that both the hysteresis cycles and the ON/OFF threshold of one polariton signal can be entirely controlled by a second polariton fluid. Furthermore, we study the ultrafast switching dynamics of a driven polariton state, demonstrating the ability to control the polariton population with an external laser pulse, in less than a few picoseconds.

cond-mat.mes-hall↗

arXiv2011

All-optical control of the quantum flow of a polariton superfluid

While photons in vacuum are massless particles that do not interact with each other, significant photon-photon interactions appear in suitable nonlinear media, leading to novel hydrodynamic behaviors typical of quantum fluids. Here we show the formation of vortex-antivortex pairs in a Bose-Einstein condensate of exciton-polaritons -a coherent gas of strongly dressed photons- flowing at supersonic speed against an artificial potential barrier created and controlled by a light beam in a planar semiconductor microcavity. The observed hydrodynamical phenomenology is in agreement with original theoretical predictions based on the Gross-Pitaevskii equation, recently generalized to the polariton context. However, in contrast to this theoretical work, we show how the initial position and the subsequent trajectory of the vortices crucially depend on the strength and size of the artificial barrier. Additionally, we demonstrate how a suitably tailored optical beam can be used to permanently trap and store the vortices that are hydrodynamically created in the wake of a natural defect. These observations are borne out by time-dependent theoretical simulations.

cond-mat.mes-hall↗

arXiv2008

Quantum-fluid dynamics of microcavity polaritons

Semiconductor microcavities offer a unique system to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons--a mixture of excitons and photons--behave, in the low density limit, as bosons that can undergo a phase transition to a regime characterised by long range coherence. Condensates of polaritons have been advocated as candidates for superfluidity; and the formation of vortices as well as elementary excitations with a linear dispersion are actively sought after. In this work, we have created and set in motion a macroscopically degenerate state of polaritons and let it collide with a variety of defects present in the sample. Our experiments show striking manifestations of a coherent light-matter packet that displays features of a superfluid, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system where it travels at ultra-fast velocity of the order of 1% the speed of light. Our main results are the observation of i) a linear polariton dispersion accompanied with diffusion-less motion, ii) flow without resistance when crossing an obstacle, iii) suppression of Rayleigh scattering and iv) splitting into two fluids when the size of the obstacle is comparable with the size of the wavepacket. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.

cond-mat.mes-hall↗

arXiv2008

Observation of long-lived polariton states in semiconductor microcavities across the parametric threshold

The excitation spectrum around the pump-only stationary state of a polariton optical parametric oscillator (OPO) in semiconductor microcavities is investigated by time-resolved photoluminescence. The response to a weak pulsed perturbation in the vicinity of the idler mode is directly related to the lifetime of the elementary excitations. A dramatic increase of the lifetime is observed for a pump intensity approaching and exceeding the OPO threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following onset of the soft Goldstone mode.

cond-mat.other↗