arXiv subjects

D. Sanvitto

Publications and source records attributed to D. Sanvitto.

50 records · Page 3Linked to original sources

arXiv2012

Frictionless flow in a binary polariton superfluid

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.

cond-mat.mes-hall↗

arXiv2011

Build up of off-diagonal long-range order in microcavity exciton-polaritons across the parametric threshold

We report an experimental study of the spontaneous spatial and temporal coherence of polariton condensates generated in the optical parametric oscillator configuration, below and at the parametric threshold, and as a function of condensate area. Above the threshold we obtain very long coherence times (up to 3 ns) and a spatial coherence extending over the entire condensate (40 μm). The very long coherence time and its dependence on condensate area and pump power reflect the suppression of polariton-polariton interactions by an effect equivalent to motional narrowing.

cond-mat.mes-hall↗

arXiv2011

Onset and dynamics of vortex-antivortex pairs in polariton OPO superfluids

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 spontaneous 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.

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↗

arXiv2011

Persistent currents and quantised vortices in a polariton superfluid

Semiconductor microcavity polaritons in the optical parametric scattering regime have been recently demonstrated to display a new variety of dissipationless superfluid behaviour. We report the first observation in resonantly pumped exciton polaritons of a metastable persistent superflow carrying quantum of angular momentum, m. The quantised vortex, excited by a weak 2 ps pulsed probe, is shown to last for at least 80 ps, limited only by the leaking outside the cavity. The polariton circulating superfluid persists in the absence of the driving rotating probe with no apparent dissipation. In addition, for a moving superfluid, we show the coherent splitting of a quantised double vortex, with charge m=2, into two singly quantised vortices of m=1. Remarkably, we observe the m=2 vortex to be stable when they are at rest. The experimental results are compared with a theoretical analysis, obtained describing the triggered parametric scattering regime of polaritons via a two-component Gross-Pitaevskii equation, including pump and decay processes.

cond-mat.other↗

arXiv2010

Linear and nonlinear coupling of quantum dots in microcavities

We discuss the topical and fundamental problem of strong-coupling between a quantum dot an the single mode of a microcavity. We report seminal quantitative descriptions of experimental data, both in the linear and in the nonlinear regimes, based on a theoretical model that includes pumping and quantum statistics.

cond-mat.mes-hall↗

arXiv2010

Propagating wave-packets and quantised currents in coherently driven polariton superfluids

We study the properties of propagating polariton wave-packets and their connection to the stability of doubly charged vortices. Wave-packet propagation and related photoluminescence spectra exhibit a rich behaviour dependent on the excitation regime. We show that, because of the non-quadratic polariton dispersion, doubly charged vortices are stable only when initiated in wave-packets propagating at small velocities. Vortices propagating at larger velocities, or those imprinted directly into the polariton optical parametric oscillator (OPO) signal and idler are always unstable to splitting.

cond-mat.quant-gas↗

arXiv2009

Collective dynamics of excitons and polaritons in semiconductor nanostructures

Time resolved photoluminescence is a powerful technique to study the collective dynamics of excitons and polaritons in semiconductor nanostructures. We present a two excitation pulses technique to induce the ultrafast and controlled quenching of the exciton emission in a quantum well. The depth of the dip is given by the magnitude of the warming of the carriers induced by the arrival of a laser pulse when an exciton population is already present in the sample. We use this technique to study the relaxation mechanisms of polaritons in semiconductor microcavities, which are of great importance to enhance the conditions for their condensation under non-resonant excitation. We also explore the dynamics of polariton fluids resonantly created in the lower polariton branch in a triggered optical parametric oscillator configuration, showing evidence of polariton superfluidity, and opening up the way to the real-time study of quantum fluids.

cond-mat.mes-hall↗

arXiv2009

Effect of pure dephasing on the Jaynes-Cummings nonlinearities

We study the effect of pure dephasing on the strong-coupling between a quantum dot and the single mode of a microcavity in the nonlinear regime. We show that the photoluminescence spectrum of the system has a robust tendency to display triplet structures, instead of the expected Jaynes-Cummings pairs of doublets at the incommensurate frequencies $\pm(\sqrt{n}\pm\sqrt{n-1})$ for integer $n$. We show that current experimental works may already manifest signatures of single photon nonlinearities.

cond-mat.mes-hall↗

arXiv2009

Dynamics of formation and decay of coherence in a polariton condensate

We study the dynamics of formation and decay of a condensate of microcavity polaritons. We investigate the relationship between the number of particles, the emission's linewidth and its degree of linear polarization which serves as the order parameter. Tracking the condensate's formation, we show that, even when interactions are negligible, coherence is not determined only by occupation of the ground state. As a result of the competition between the coherent and thermal fractions of the condensate, the highest coherence is obtained some time after the particle number has reached its maximum.

cond-mat.mes-hall↗

arXiv2009

Exciton-polariton condensation in a natural two dimensional trap

Bose Einstein condensation of exciton-polaritons has recently been reported in homogeneous structures only affected by random in-plane fluctuations. We have taken advantage of the ubiquitous defects in semiconductor microcavities to reveal the spontaneous dynamical condensation of polaritons in the quantised levels of a trap. We observe condensation in several quantized states, taking their snapshots in real and reciprocal space. We show also the effect of particle interactions for high occupations numbers, revealed by a change in the confined wave function toward the Thomas Fermi profile.

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↗

arXiv2006

Single-photon nonlinearity of a semiconductor quantum dot in a cavity

A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED). The strong coupling regime between the atom and cavity-confined photon corresponds to the reversible exchange of energy between the two modes, and underpins a wide range of CQED phenomena with applications in quantum information science, including for example as quantum logic gates and as sources of entangled states. An important advance was achieved recently when strong coupling between excitons and cavity photons was reported for the first time for localized quantum dots (QDs) in micron-size solid state cavities. This has significance in terms of scalability and integration with other optical devices, and could lead to the emergence of "quantum optics on a chip" technology. However the results presented so far for quantum dots are in the linear regime, corresponding to coupling to the vacuum field (vacuum Rabi splitting); they are not a true QED effect and can equally well be described by classical physics as the coupling between two oscillators. In this paper, we present evidence for a purely quantum phenomenon for the QD/cavity photon system, namely the increase in splitting of the levels when the mean number of photons in the cavity is increased. This corresponds to non-linearities on the single-photon scale: the presence of a single excitation in the cavity changes the level structure, affecting the emission energies for a second photon. Such results are a first step in demonstrating the promise of quantum dots for CQED applications.

cond-mat.mes-hall↗