Observation of inversion, hysteresis, and collapse of spin in optically trapped polariton condensates

The spin and intensity of optically trapped polariton condensates are studied under steady-state elliptically-polarised nonresonant pumping. Three distinct effects are observed: (1) spin inversion where condensation occurs in the opposite handedness from the pump, (2) spin/intensity hysteresis as the pump power is scanned, and (3) a sharp ‘spin collapse’ transition in the condensate spin as a function of the pump ellipticity. We show these effects are strongly dependent on trap size and sample position and are linked to small counterintuitive energy differences between the condensate spin components. Our results, which fail to be fully described within the commonly used nonlinear equations for polariton condensates, show that a more accurate microscopic picture is needed to unify these phenomena in a two-dimensional condensate theory

Nonresonant optical control of a spinor polariton condensate

We investigate the spin dynamics of polariton condensates spatially separated from and effectively confined by the pumping exciton reservoir. We obtain a strong correlation between the ellipticity of the non-resonant optical pump and the degree of circular polarisation (DCP) of the condensate at the onset of condensation. With increasing excitation density we observe a reversal of the DCP. The spin dynamics of the trapped condensate are described within the framework of the spinor complex Ginzburg-Landau equations in the Josephson regime, where the dynamics of the system are reduced to a current-driven Josephson junction. We show that the observed spin reversal is due to the interplay between an internal Josephson coupling effect and the detuning of the two projections of the spinor condensate via transition from a synchronised to a desynchronised regime. These results suggest that spinor polariton condensates can be controlled by tuning the non-resonant excitation density offering applications in electrically pumped polariton spin switches.P.G.L acknowledges support by the Engineering and Physical Sciences Research Council of UK through the Hybrid Polaritonics Programme Grant (EP/M025330/1). P.G.S. acknowledges funding from the EU Social Fund and Greek National Resources (EPEAEK II, HRAKLEITOS II), N.G.B acknowledges the financial support by Ministry of Education and Science of Russian Federation 1425320 (Project DOI: RFMEFI58114X0006). The authors acknowledge fruitful discussions with Prof. Alexey Kavokin and Dr Hamid Ohadi. The data from this paper can be obtained from the University of Southampton e-Print research repository.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by APS

Spontaneous spin bifurcations and ferromagnetic phase transitions in a spinor exciton-polariton condensate

We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatially trapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two elliptically polarized (up to 95% circularly-polarized) states with opposite handedness of polarization. The magnetized condensate remains stable for many seconds at 5 K, but at higher temperatures it can flip from one magnetic orientation to another. We optically address these states and demonstrate the inversion of the magnetic state by resonantly injecting 100-fold weaker pulses of opposite spin. Theoretically, these phenomena can be well described as spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations.This work was supported by Grants EPSRC No. EP/G060649/1, EU No. CLERMONT4 235114, EU No. INDEX 289968, Spanish MEC (MAT2008-01555), Greek GSRT ARISTEIA Apollo program and Fundación La Caixa, and Mexican CONACYT No. 251808. FP acknowledges financial support through an EPSRC doctoral prize fellowship at the University of Cambridge and a Schrödinger fellowship at the University of Oxford.This is the final version of the article. It first appeared from the American Physical Society via http://dx.doi.org/10.1103/PhysRevX.5.03100

Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire

We show that a single exciton state in an individual ordered conjugated polymer chain exhibits macroscopic quantum spatial coherence reaching tens of microns, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.Comment: 12 pages with 2 figure

High-angle optically accessible Brewster cavity exciton-polaritons

We report on the observation of the strong-coupling regime between quantum well excitons and a high incidence “Brewster cavity mode” previously identified as the generalized Brewster angle condition in multilayer structures [H. F. Mahlein, J. Opt. Soc. Am. 64, 647 (1974)]. This propagating mode is inside the light cone and therefore can be accessed from the top side of the sample without the need for prism or grating coupling methods. The observed anticrossing is clear evidence of the strong light-matter coupling regime. All the results are accurately reproduced by transfer matrix simulations. These results demonstrate the high potential of such structures for the study of propagating polaritons at high k, which could be harnessed for the realization of polaritonic circuit devices.Financial support from the bilateral Greece-Russia Polisimulator project cofinanced by Greece and the EU Regional Development Fund, Russian Science Foundation Grants No. 19-72-20120, UK EPSRC EP/L027151/1, EP/N016920/1 grants are acknowledged. The research was cofinanced by Greece and EU-ESF Fund through HRDELL 2014-2020 program (project MIS 5004464)

Spin Order and Phase Transitions in Chains of Polariton Condensates

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice.We acknowledge Grants No. EPSRC EP/L027151/1, No. EU INDEX 289968, No. ERC “POLAFLOW” Starting Grant, ERC LINASS 320503, Spanish MEC (MAT2008- 01555), Mexican CONACYT 251808, Leverhulme Trust Grant No. VP1-2013-011 and Fundación La Caixa. H. S. and I. S. acknowledge support by the Research Fund of the University of Iceland, The Icelandic Research Fund, Grant No. 163082-051. T. L. was supported by the MOE AcRF Tier 1 Grant No. 2016-T1-001-084. P. S. acknowledges financial support from the Stavros Niarchos Foundation, “ARCHERS” project

Tuning the Energy of a Polariton Condensate via Bias-Controlled Rabi Splitting

We introduce an electrically driven scheme to tune the polariton condensate energy in a high-finesse GaAs microcavity. In contrast to the conventional redshift observed in semiconductor quantum wells (QWs) under applied electrical bias arising from the quantum-confined Stark effect (QCSE), we report here the blueshift of a polariton condensate caused by controlled reduction of the Rabi splitting due to tunneling-induced charge buildup and fractional bleaching of QWs. At larger electrical bias, the QCSE becomes dominant, leading to a redshift in the linear regime, while in the nonlinear regime to the eventual quenching of the condensate emission. This ability to tune the polariton condensate energy brings within reach the realization of voltage-controlled polariton condensate devices and variable-wavelength sources of coherent light

Sculpting oscillators with light within a nonlinear quantum fluid

Seeing macroscopic quantum states directly remains an elusive goal. Particles with boson symmetry can condense into such quantum fluids producing rich physical phenomena as well as proven potential for interferometric devices [1-10]. However direct imaging of such quantum states is only fleetingly possible in high-vacuum ultracold atomic condensates, and not in superconductors. Recent condensation of solid state polariton quasiparticles, built from mixing semiconductor excitons with microcavity photons, offers monolithic devices capable of supporting room temperature quantum states [11-14] that exhibit superfluid behaviour [15,16]. Here we use microcavities on a semiconductor chip supporting two-dimensional polariton condensates to directly visualise the formation of a spontaneously oscillating quantum fluid. This system is created on the fly by injecting polaritons at two or more spatially-separated pump spots. Although oscillating at tuneable THz-scale frequencies, a simple optical microscope can be used to directly image their stable archetypal quantum oscillator wavefunctions in real space. The self-repulsion of polaritons provides a solid state quasiparticle that is so nonlinear as to modify its own potential. Interference in time and space reveals the condensate wavepackets arise from non-equilibrium solitons. Control of such polariton condensate wavepackets demonstrates great potential for integrated semiconductor-based condensate devices.Comment: accepted in Nature Physic

Vortices in polariton OPO superfluids

This chapter reviews the occurrence of quantised vortices in polariton fluids, primarily when polaritons are driven in the optical parametric oscillator (OPO) regime. We first review the OPO physics, together with both its analytical and numerical modelling, the latter being necessary for the description of finite size systems. Pattern formation is typical in systems driven away from equilibrium. Similarly, we find that uniform OPO solutions can be unstable to the spontaneous formation of quantised vortices. However, metastable vortices can only be injected externally into an otherwise stable symmetric state, and their persistence is due to the OPO superfluid properties. We discuss how the currents charactering an OPO play a crucial role in the occurrence and dynamics of both metastable and spontaneous vortices.Comment: 40 pages, 16 figure

Synchronization crossover of polariton condensates in weakly disordered lattices

We demonstrate that the synchronization of a lattice of solid-state condensates when intersite tunneling is switched on depends strongly on the weak local disorder. This finding is vital for implementation of condensate arrays as computation devices. The condensates here are nonlinear bosonic fluids of exciton-polaritons trapped in a weakly disordered Bose-Hubbard potential, where the nearest-neighboring tunneling rate (Josephson coupling) can be dynamically tuned. The system can thus be tuned from a localized to a delocalized fluid as the number density or the Josephson coupling between nearest neighbors increases. The localized fluid is observed as a lattice of unsynchronized condensates emitting at different energies set by the disorder potential. In the delocalized phase, the condensates synchronize and long-range order appears, evidenced by narrowing of momentum and energy distributions, new diffraction peaks in momentum space, and spatial coherence between condensates. Our paper identifies similarities and differences of this nonequilibrium crossover to the traditional Bose-glass to superfluid transition in atomic condensates