L. Viña

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.

Persistent currents and quantized vortices in a polariton superfluid

Similar to atoms in cold gases, exciton–polaritons in semiconductor microcavities can undergo Bose–Einstein condensation, but under non-equilibrium conditions. Now, quantized vortices and persistent currents — hallmarks of superfluid behaviour — have been observed in such condensates.

AlGaAs/GaAs(111) heterostructures grown by molecular beam epitaxy

We have grown AlGaAs/GaAs heterostructures on (111) oriented GaAs substrates by molecular beam epitaxy. Materials with good optical and electrical properties, including mobility enhancement in two‐dimensional electron and hole gases, have been obtained for the first time.

Polarization control of the nonlinear emission of semiconductor microcavities.

The degree of circular polarization ( Weierstrass p ) of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards K approximately 0 cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the +/-1 spin levels at K approximately 0. At short times after a pulsed excitation Weierstrass p reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy (+1/-1 spin for positive/negative detuning).

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. For intrinsic wells, whose optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. For doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.

Stark shifts in GaAs/GaAlAs quantum wells studied by photoluminescence spectroscopy

The authors have measured, by means of low-temperature photoluminescence spectroscopy, the dependence on the well thickness of the shift of the excitonic recombination in GaAs/GaAlAs quantum wells induced by an external electric field. With increasing thickness, up to 230 AA, an increasing Stark shift, amounting to more than 100 meV for an external field of approximately 105 V cm-1, has been found. A corresponding quenching of the luminescence intensity has also been measured. Theoretical calculations of the eigenstates have been carried out and shown to compare favourably with the experiments. Two different approaches, numerical solution of the Schrodinger equation and variational calculations of the eigenstates, yield similar results for the Stark shift. Using the latter method, the quenching of the photoluminescence is correlated with the decrease in the wavefunction overlap between electrons and holes, as they are spatially separated by the field.

High‐purity GaAs grown by molecular‐beam epitaxy

We have grown high‐purity GaAs on (100), (311)A, and (311)B orientations by molecular‐beam epitaxy (MBE). While undoped GaAs grown on (100) and (311)A are typically p type, growth on (311)B orientation has yielded n type with a liquid‐nitrogen electron mobility of 1.3×105 cm2 V−1 s−1, which is among the highest mobilities reported for MBE‐grown materials. Low‐temperature photoluminescence showed well‐resolved impurity bound exciton peaks consistent with electrical results. The possible incorporations of impurities, especially carbon, are discussed. Our work demonstrates that the previously reported undoped p‐type GaAs(100) are compensated.

SUBSTRATE EFFECT ON CDTE LAYERS GROWN BY METALORGANIC VAPOR PHASE EPITAXY

CdTe layers were grown by metalorganic vapor phase epitaxy (MOVPE) on different substrates like sapphire, GaAs, and CdTe wafers. The growth was carried out at the temperature 340 °C and time in the range of 2–4 h using dimethyl-cadmium and diisopropil-tellurium as precursors. The layers were studied by scanning electron microscopy, Rutherford backscattering spectroscopy, and high resolution low-temperature photoluminescence spectroscopy. The surface morphology and RBS and PL spectra of CdTe MOVPE layers are reported and the substrate effect on the layer properties is demonstrated.

Effect of interactions on vortices in a nonequilibrium polariton condensate.

We demonstrate the creation of vortices in a macroscopically occupied polariton state formed in a semiconductor microcavity. A weak external laser beam carrying orbital angular momentum (OAM) is used to imprint a vortex on the condensate arising from the polariton optical parametric oscillator (OPO). The vortex core radius is found to decrease with increasing pump power, and is determined by polariton-polariton interactions. As a result of OAM conservation in the parametric scattering process, the excitation consists of a vortex in the signal and a corresponding antivortex in the idler of the OPO. The experimental results are in good agreement with a theoretical model of a vortex in the polariton OPO.

Polarized interacting exciton gas in quantum wells and bulk semiconductors

We develop a theory to calculate exciton binding energies of both two- and three-dimensional spin polarized exciton gases within a mean field approach. Our method allows the analysis of recent experiments showing the importance of the polarization and intensity of the excitation light on the exciton luminescence of GaAs quantum wells. We study the breaking of the spin degeneracy observed at high exciton density