Phase space monitoring of exciton-polariton multistability

Dynamics of exciton-polariton multistability is theoretically investigated. Phase portraits are used as a tool to enlighten the microscopic phenomena which influence spin multistability of a confined polariton field as well as ultrafast reversible spin switching. The formation of a non-radiative reservoir, due to polariton pairing into biexcitons is found to play the lead role in the previously reported spin switching experiments. Ways to tailor this reservoir formation are discussed in order to obtain optimal spin switching reliability

Selective photoexcitation of exciton-polariton vortices

We resonantly excite exciton-polariton states confined in cylindrical traps. Using a homodyne detection setup, we are able to image the phase and amplitude of the confined polariton states. We evidence the excitation of vortex states, carrying an integer angular orbital momentum m, analogous to the transverse TEM01* "donut" mode of cylindrically symmetric optical resonators. Tuning the excitation conditions allows us to select the charge of the vortex. In this way, the injection of singly charged (m = 1 & m = -1) and doubly charged (m = 2) polariton vortices is shown. This work demonstrates the potential of in-plane confinement coupled with selective excitation for the topological tailoring of polariton wavefunctions

Exciton-polaritons gas as a nonequilibrium coolant

Using angle-resolved Raman spectroscopy, we show that a resonantly excited ground-state exciton-polariton fluid behaves like a nonequilibrium coolant for its host solid-state semiconductor microcavity. With this optical technique, we obtain a detailed measurement of the thermal fluxes generated by the pumped polaritons. We thus find a maximum cooling power for a cryostat temperature of $50$K and below where optical cooling is usually suppressed, and we identify the participation of an ultrafast cooling mechanism. We also show that the nonequilibrium character of polaritons constitutes an unexpected resource: each scattering event can remove more heat from the solid than would be normally allowed using a thermal fluid with normal internal equilibration.Comment: 5 pages, 3 figures + supplemental materia

Dynamics of dark-soliton formation in a polariton quantum fluid

Polariton fluids have revealed huge potentialities in order to investigate the properties of bosonic fluids at the quantum scale. Among those properties, the opportunity to create dark as well as bright solitons has been demonstrated recently. In the present experiments, we image the formation dynamics of oblique dark solitons. They nucleate in the wake of an engineered attractive potential that perturbs the polariton quantum fluid. Thanks to time and phase measurements, we assess quantitatively the formation process. The formation velocity is observed to increase with increasing distance between the flow injection point and the obstacle which modulates the density distribution of the polariton fluid. We propose an explanation in terms of the increased resistance to the flow and of the conditions for the convective instability of dark solitons. By using an iterative solution of the generalized Gross-Pitaevskii equation, we are able to reproduce qualitatively our experimental results

Phase-resolved imaging of confined exciton-polariton wave functions in elliptical traps

We study the wave functions of exciton polaritons trapped in the elliptical traps of a patterned microcavity. A homodyne detection setup with numerical off-axis filtering allows us to retrieve the amplitude and the phase of the wave functions. Doublet states are observed as the result of the ellipticity of the confinement potential and are successfully compared to even and odd solutions of Mathieu equations. We also show how superpositions of odd and even states can be used to produce "donut" and "eight-shape" states which can be interpreted as polariton vortices

VCSEL Based on InAs Quantum-Dashes With a Lasing Operation Over a 117-nm Wavelength Span

International audienceWe report an InP based vertical cavity surface emitting laser (VCSEL) achieving a lasing operation between 1529 and 1646 nm. This optically-pumped VCSEL includes a wide-gain bandwidth active region based on InAs quantum dashes and wideband dielectric Bragg mirrors. Based on a wedge microcavity design, we obtain a spatial dependence of the resonant wavelength along the wafer, enabling thus to monitor the gain material bandwidth. We demonstrate a 117 nm continuous wavelength variation of the VCSEL emission, a consequence of the important and wide gain afforded by the use of optimized quantum dashes

VCSEL à fils quantiques présentant une émission laser de 1647 à 1542 nm

National audienceLes lasers à cavité verticale émettant par la surface (VCSEL) présentent de grands intérêts pour des applications variées (télécommunication, capteurs, ..), d'autant plus si ces derniers s'avèrent stables et accordables en longueur d'onde. Au-delà du procédé technologique utilisé, cette dernière propriété est aussi très limitée par l'extension du gain spectral de la zone active. Nous présentons ici la réalisation d'un VCSEL émettant à 1.55 μm, et présentant une émission laser sur une plage en longueur d'onde de 105 nm

VCSEL à fils quantiques présentant une émission laser de 1647 à 1542 nm

National audienceLes lasers à cavité verticale émettant par la surface (VCSEL) présentent de grands intérêts pour des applications variées (télécommunication, capteurs, ..), d'autant plus si ces derniers s'avèrent stables et accordables en longueur d'onde. Au-delà du procédé technologique utilisé, cette dernière propriété est aussi très limitée par l'extension du gain spectral de la zone active. Nous présentons ici la réalisation d'un VCSEL émettant à 1.55 μm, et présentant une émission laser sur une plage en longueur d'onde de 105 nm

Hydrodynamic nucleation of quantized vortex pairs in a polariton quantum fluid

Quantized vortices appear in quantum gases at the breakdown of superfluidity. In liquid helium and cold atomic gases, they have been indentified as the quantum counterpart of turbulence in classical fluids. In the solid state, composite light-matter bosons known as exciton polaritons have enabled studies of non-equilibrium quantum gases and superfluidity. However, there has been no experimental evidence of hydrodynamic nucleation of polariton vortices so far. Here we report the experimental study of a polariton fluid flowing past an obstacle and the observation of nucleation of quantized vortex pairs in the wake of the obstacle. We image the nucleation mechanism and track the motion of the vortices along the flow. The nucleation conditions are established in terms of local fluid density and velocity measured on the obstacle perimeter. The experimental results are successfully reproduced by numerical simulations based on the resolution of the Gross-Pitaevskii equation

Photoelectrochemical water oxidation of GaP 1−x Sb x with a direct band gap of 1.65 eV for full spectrum solar energy harvesting

International audienceHydrogen produced using artificial photosynthesis, i.e. solar splitting of water, is a promising energy alternative to fossil fuels. Efficient solar water splitting demands a suitable band gap to absorb near full spectrum solar energy and a photoelectrode that is stable in strongly alkaline or acidic electrolytes. In this work, we demonstrate for the first time, a perfectly relaxed GaP0.67Sb0.33 monocrystalline alloy grown on a silicon substrate with a direct band gap of 1.65 eV by molecular beam epitaxy (MBE) without any evidence of chemical disorder. Under one Sun illumination, the GaP0.67Sb0.33 photoanode with a 20 nm TiO2 protective layer and 8 nm Ni co-catalyst layer shows a photocurrent density of 4.82 mA cm−2 at 1.23 V and an onset potential of 0.35 V versus the reversible hydrogen electrode (RHE) in 1.0 M KOH (pH = 14) aqueous solution. The photoanode yields an incident-photon-to-current efficiency (IPCE) of 67.1% over the visible range between wavelengths 400 nm to 650 nm. Moreover, the GaP0.67Sb0.33 photoanode was stable over 5 h without degradation of the photocurrent under strong alkaline conditions under continuous illumination at 1 V versus RHE. Importantly, the direct integration of the 1.65 eV GaP0.67 Sb0.33 on 1.1 eV silicon may pave the way for an ideal tandem photoelectrochemical system with a theoretical solar to hydrogen efficiency of 27%