M. Wouters

Multistability of a coherent spin ensemble in a semiconductor microcavity

Coherent manipulation of spin ensembles is a key issue in the development of spintronics. In particular, multivalued spin switching may lead to new schemes of logic gating and memories. This phenomenon has been studied with atom vapours 30 years ago, but is still awaited in the solid state. Here, we demonstrate spin multistability with microcavity polaritons in a trap. Owing to the spinor nature of these light-matter quasiparticles and to the anisotropy of their interactions, we can optically control the spin state of a single confined level by tuning the excitation power, frequency and polarization. First, we realize high-efficiency power-dependent polarization switching. Then, at constant excitation power, we evidence polarization hysteresis and determine the conditions for realizing multivalued spin switching. Finally, we demonstrate an unexpected regime, where our system behaves as a high-contrast spin trigger. These results open new pathways to the development of advanced spintronics devices and to the realization of multivalued logic circuits.

Coherent Oscillations in an Exciton-Polariton Josephson Junction

We report on the observation of spontaneous coherent oscillations in a microcavity polariton bosonic Josephson junction. Condensation of exciton polaritons here takes place under incoherent excitation in a double potential well naturally formed in the disorder. Coherent oscillations set on at an excitation power well above the condensation threshold. The time resolved population and phase dynamics reveal the analogy with the ac Josephson effect. A theoretical two-mode model describes the observed effects, explaining how the different realizations of the pulsed experiment can be in phase.

Probing the Dynamics of Spontaneous Quantum Vortices in Polariton Superfluids

The experimental investigation of spontaneously created vortices is of utmost importance for the understanding of quantum phase transitions towards a superfluid phase, especially for two-dimensional systems that are expected to be governed by the Berezinski-Kosterlitz-Thouless physics. By means of time-resolved near-field interferometry we track the path of such vortices, created at random locations in an exciton-polariton condensate under pulsed nonresonant excitation, to their final pinning positions imposed by the stationary disorder. We formulate a theoretical model that successfully reproduces the experimental observations.

Coexisting nonequilibrium condensates with long-range spatial coherence in semiconductor microcavities

Real- and momentum-space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under nonresonant excitation using a laser source with reduced intensity fluctuations on the time scale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower-energy modes are strongly localized by the spatial polaritonic potential disorder on a scale of a few microns. Higher-energy modes have finite k vectors and are delocalized over 10– 15 m. All the modes exhibit long-range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local polaritonic potential, and the reduced intensity fluctuations of the excitation laser.

Synchronized and desynchronized phases of coupled nonequilibrium exciton-polariton condensates

We theoretically analyze the synchronized and desynchronized phases of coupled nonequilibrium polariton condensates within mean field theory. An analytical condition for the existence of a synchronized phase is derived for two coupled wells. The case of many wells in a two-dimensional disordered geometry is studied numerically. The relation to recent experiments on polariton condensation in CdTe microcavities is discussed.

Energy relaxation in one-dimensional polariton condensates

We study the kinetics of polariton condensation accounting for the condensation process as well as the energy relaxation of condensed polaritons due to their scattering with phonons and excitons. By assuming a Boltzmann kinetic description of the scattering process, we show that intracondensate relaxation can be accounted for by an additional time-dependent term in the Gross-Pitaevskii equation. As an example, we apply the formalism to the experimental results recently obtained in polariton microwires [E. Wertz, L. Ferrier, D. Solnyshkov, R. Johne, D. Sanvitto, A. Lemaitre, I. Sagnes, R. Grousson, A. V. Kavokin, P. Senellart, G. Malpuech, and J. Bloch, Nat. Phys. 6, 860 (2010)]. In the presence of a local nonresonant optical pump, a dynamic balance between spatially dependent relaxation and particle loss develops and excites a series of modes, roughly equally spaced in energy. Upon comparison, excellent agreement is found with the experimental data.

Stochastic classical field model for polariton condensates

We use the truncated Wigner approximation to derive stochastic classical field equations for the description of polariton condensates. Our equations are shown to reduce to the Boltzmann equation in the limit of low polariton density. Monte Carlo simulations are performed to analyze the momentum distribution and the first- and second-order coherences when the particle density is varied across the condensation threshold.

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 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 optical parametric oscillator threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following appearance of a soft Goldstone mode in the spectrum.

Superfluid to Bose-Glass Transition in a 1D Weakly Interacting Bose Gas

We study the one-dimensional Bose gas in spatially correlated disorder at zero temperature, using an extended density-phase Bogoliubov method. We analyze, in particular, the decay of the one-body density matrix and the behavior of the Bogoliubov excitations across the phase boundary. We observe that the transition to the Bose-glass phase is marked by a power-law divergence of the density of states at low energy. A measure of the localization length displays a power-law energy dependence in both regions, with the exponent equal to -1 at the boundary. We draw the phase diagram of the superfluid-insulator transition in the limit of small interaction strength.

Exciton-polariton Bose-Einstein condensation: advances and issues

In this review, we present a comprehensive set of experimental results on microcavity-polariton Bose-Einstein Condensation (BEC), obtained within a close collaboration between Institut Neel, Grenoble, France and EPFL, Lausanne, Switzerland. First, we recall the main observations, i.e., massive occupation of the ground state and build-up of long range order, which led us to conclude that polariton BEC indeed occurs. Then, the highly disordered environment in which the condensation takes place is considered: we show how interactions are a necessary ingredient for polariton BEC. Finally we discuss quantised vortices observed for the first time in polariton condensates. Their unusual features are shown to be inherited from the disordered environment and the driven-dissipative character of the polariton BEC.