M. D. Martín

Collective fluid dynamics of a polariton condensate in a semiconductor microcavity

Semiconductor microcavities offer unique systems in which to investigate the physics of weakly interacting bosons. Their elementary excitations, polaritons—mixtures of excitons and photons—can accumulate in macroscopically degenerate states to form various types of condensate in a wide range of experimental configurations, under either incoherent or coherent excitation. Condensates of polaritons have been put forward as candidates for superfluidity, and the formation of vortices as well as elementary excitations with linear dispersion are actively sought as evidence to support this. Here, using a coherent excitation triggered by a short optical pulse, we have created and set in motion a macroscopically degenerate state of polaritons that can be made to collide with a variety of defects present in the microcavity. Our experiments show striking manifestations of a coherent light–matter packet, travelling at high speed (of the order of one per cent of the speed of light) and displaying collective dynamics consistent with superfluidity, although one of a highly unusual character as it involves an out-of-equilibrium dissipative system. Our main results are the observation of a linear polariton dispersion accompanied by diffusionless motion; flow without resistance when crossing an obstacle; suppression of Rayleigh scattering; and splitting into two fluids when the size of the obstacle is comparable to the size of the wave packet. This work opens the way to the investigation of new phenomenology of out-of-equilibrium condensates.eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website.

Interplay of exciton and electron-hole plasma recombination on the photoluminescence dynamics in bulk GaAs

We present a systematic study of the exciton and electron-hole plasma photoluminescence dynamics in bulk GaAs for various lattice temperatures and excitation densities. The competition between the exciton and electron-hole pair recombination dominates the onset of the luminescence. We show that the metal-to-insulator transition, induced by temperature and/or excitation density, can be directly monitored by the carrier dynamics and the time-resolved spectral characteristics of the light emission. The dependence on carrier density of the photoluminescence rise time is strongly modified around a lattice temperature of 49 K, corresponding to the exciton binding energy 4.2 meV. In a similar way, the rise-time dependence on lattice temperature undergoes a relatively abrupt change at an excitation density of 120–180 10 15 cm �3 , which is about five times greater than the calculated Mott density in GaAs taking into account many body corrections.

Motion of spin polariton bullets in semiconductor microcavities

The dynamics of optical switching in semiconductor microcavities in the strong coupling regime is studied by using time- and spatially resolved spectroscopy. The switching is triggered by polarized short pulses which create spin bullets of high polariton density. The spin packets travel with speeds of the order of 10(6)  m/s due to the ballistic propagation and drift of exciton polaritons from high to low density areas. The speed is controlled by the angle of incidence of the excitation beams, which changes the polariton group velocity.

Optical anisotropy and pinning of the linear polarization of light in semiconductor microcavities

We report strong experimental evidence of the optical anisotropy in a CdTe-based microcavity: the polarization of light is pinned to one of the crystallographic axes independently of the polarization of the excitation. The polarization degree depends strongly on the excitation power, reaching almost 100% in the stimulated regime. The relaxation time of the polarization is about 1 ns. We argue that all of this is an effect of a splitting of the polariton doublet at k = 0. We consider different sources for the splitting and conclude that the most likely one is optical birefringence in the mirrors and/or the cavity.

Spin selective filtering of polariton condensate flow

Spin-selective spatial filtering of propagating polariton condensates, using a controllable spin-dependent gating barrier, in a one-dimensional semiconductor microcavity ridge waveguide is reported. A nonresonant laser beam provides the source of propagating polaritons, while a second circularly polarized weak beam imprints a spin dependent potential barrier, which gates the polariton flow and generates polariton spin currents. A complete spin-based control over the blocked and transmitted polaritons is obtained by varying the gate polarization.

Energy relaxation of exciton-polariton condensates in quasi-one-dimensional microcavities

C.A. and G.T. acknowledge financial support from Spanish FPU and FPI scholarships, respectively. P.S. acknowledges Greek GSRT program “ARISTEIA” (1978) for financial support. The work was partially supported by the Spanish MEC MAT2011-22997, CAM (S-2009/ESP-1503), and FP7 ITN’s “Clermont4” (235114), “Spin-optronics” (237252), and INDEX (289968) projects

Polariton and spin dynamics in semiconductor microcavities under non-resonant excitation

Semiconductor microcavities offer an ideal scenario to study strong radiation-matter interactions. In this paper we review the temporal dynamics of polaritons in II-VI and III-V based microcavities under non-resonant excitation conditions. We present evidence of final-state stimulated scattering and discuss the spin-dependent emission, which exhibits a remarkably rich behaviour.

Optical control of spin textures in quasi-one-dimensional polariton condensates

C.A. acknowledges financial support from the Spanish FPU scholarship. P.S. acknowledges the Greek GSRT program “ARISTEIA” (1978) and EU ERC “Polaflow” for financial support. The work was partially supported by the SpanishMEC MAT2011-22997 and EU-FP7 ITN INDEX(289968) projects

Operation speed of polariton condensate switches gated by excitons

C.A. acknowledges financial support from Spanish FPU scholarships. P.S. acknowledges GreekGSRT program “ARISTEIA (1978) and EU ERC “Polaflow” for financial support. The work was partially supported by the Spanish MEC MAT2011-22997, CAM (S-2009/ESP-1503), and INDEX (289968) projects

Role of supercurrents on vortices formation in polariton condensates

Observation of quantized vortices in non-equilibrium polariton condensates has been reported either by spontaneous formation and pinning in the presence of disorder or by imprinting them onto the signal or idler of an optical parametric oscillator (OPO). Here, we report a detailed analysis of the creation and annihilation of polariton vortex-antivortex pairs in the signal state of a polariton OPO by means of a short optical Gaussian pulse at a certain finite pump wave-vector. A time-resolved, interferometric analysis of the emission allows us to extract the phase of the perturbed condensate and to reveal the dynamics of the supercurrents created by the pulsed probe. This flow is responsible for the appearance of the topological defects when counter-propagating to the underlying currents of the OPO signal.