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Register a new userExciton-polariton condensates near the Dirac point in a triangular lattice
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Dirac particles, massless relativistic entities, obey linear energy dispersions and hold important implication in particle physics. Recent discovery of Dirac fermions in condensed matter systems including graphene and topological insulators raises great interests to explore relativistic properties associated with Dirac physics in solid-state materials. In addition, there are stimulating research activities to engineer Dirac paricles to eludicte their physical properties in a controllable setting. One of the successful platforms is the ultracold atom-optical lattice system, whose dynamics can be manipulated in a clean environment. A microcavity exciton-polariton-lattice system provides an alternative route with an advantage of forming high-orbital condensation in non-equilibrium conditions, which enables to explore novel quantum orbital order in two dimensions. Here we directly map the liner dispersions near the Dirac points, the vertices of the first hexagonal Brillouin zone from exciton-polariton condensates trapped in a triangular lattice. The associated velocity values are ~ 0.9 - 2*10^8 cm/s, which are consistent with the theoretical estimate 1*10^8 cm/s with a 2 mu m-lattice constant. We envision that the exciton-polariton condensates in lattices would be a promising solid-state platform, where the system order parameter can be accesses in both real and momentum spaces. We furthermore explore unique phenomena revealing quantum bose nature such as superfluidity and distinct features analogous to quantum Hall effect pertinent to time-reversal symmetry.
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We theoretically explore nonresonantly pumped polaritonic graphene, a system consisting of a honeycomb lattice of micropillars in the regime of strong light-matter coupling. We demonstrate that, depending on the parameters of the structure, such as intensity of the pump and coupling strength between the pillars, the system shows rich variety of macroscopic ordering, including analogs of ferromagnetic, antiferromagnetic, and resonant valence bond phases. Transitions between these phases are associated with dramatic reshaping of the spectrum of the system connected with spontaneous appearance of topological order.
We examine the photoluminescence of highly-excited exciton-polariton condensates in semiconductor microcavities. Under strong pumping, exciton-polariton condensates have been observed to undergo a lasing transition where strong coupling between the excitons and photons is lost. We discuss an alternative high-density scenario, where the strong coupling is maintained. We find that the photoluminescence smoothly transitions between the lower polariton energy to the cavity photon energy. An intuitive understanding of the change in spectral characteristics is given, as well as differences to the photoluminescence characteristics of the lasing case.
Recently a new type of system exhibiting spontaneous coherence has emerged -- the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The lifetime of the polaritons are typically comparable to or shorter than thermalization times, making them possess an inherently non-equilibrium nature. Nevertheless, they display many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions of what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus upon several physical phenomena exhibited by exciton-polariton condensates. In particular we examine topics such as the difference between a polariton BEC, a polariton laser, and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, BKT (Berezinskii-Kosterlitz-Thouless) and BCS (Bardeen-Cooper-Schrieffer) physics. We also discuss the physics and applications of engineered polariton structures.
Using the Feynman-Dyson diagram technique, we study nonlinear polariton-polariton scattering in a two-dimensional micropillar-based optical superlattice with hexagonal symmetry. We demonstrate that both the emerging polariton chirality and the loop Feynman diagrams up to infinite order should be strictly accounted for in the evaluation of the self-energy of the system. Further, we explicitly show that in such a design the time of polariton scattering towards the Dirac points can be drastically decreased which can be used, for instance, in engineering novel classes of polariton lasers with substantially reduced thresholds.
We demonstrate theoretically the spontaneous formation of a stochastic polarization in exciton-polariton Bose-Einstein condensates in planar microcavities under pulsed excitation. Below the threshold pumping intensity (dependent on the polariton life-time) the average polarization degree is close to zero, whilst above threshold the condensate acquires a polarization described by a (pseudospin) vector with random orientation, in general. We establish the link between second order coherence of the polariton condensate and the distribution function of its polarization. We examine also the mechanisms of polarization dephasing and relaxation.
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