Polariton Condensates In Microcavities Research Articles

Unidirectional vortex waveguides and multistable vortex pairs in polariton condensates.

Vortices carrying quantized topological charges have potential applications in information processing. In this work, we investigate vortex carriers and waveguides in microcavity polariton condensates, nonresonantly excited by a homogeneous pump with intensity grooves. An intensity groove with a ring shape in the pump gives rise to dark-ring states of the condensate with a π-phase jump, akin to dark solitons. The dark-ring states can be destroyed by a stronger density of the surrounding condensate and reduce into vortex-antivortex pairs. Multiple vortex-pair states are found to be stable in the same dark ring of the pump. When the pump ring is broader, higher-order dark states with multiple π-phase jumps can be obtained, and interestingly they can be used to construct vortex waveguides. If a single vortex is imprinted in such waveguides, it can travel in a particular direction, showing one-way transportation. In other words, an imprinted vortex with a certain charge in a specifically designed higher-order dark state is only allowed to propagate unidirectionally.

Switching Off a Microcavity Polariton Condensate near the Exceptional Point

Switching Off a Microcavity Polariton Condensate near the Exceptional Point

Circular polarization reversal of half-vortex cores in polariton condensates

Vortices are topological objects carrying quantized orbital angular momentum and have been widely studied in many physical systems for their applicability in information storage and processing. In systems with spin degree of freedom the elementary excitations are so called half-vortices, carrying a quantum rotation only in one of the two spin components. We study the spontaneous formation and stability of localized such half-vortices in semiconductor microcavity polariton condensates, non-resonantly excited by a linearly polarized ring-shaped pump. The TE-TM splitting of optical modes in the microcavity system leads to an effective spin-orbit coupling, resulting in solutions with discrete rotational symmetry. The cross-interaction between different spin components provides an efficient method to realize all-optical half-vortex core switching inverting its circular polarization state. This switching can be directly measured in the polarization resolved intensity in the vortex core region and it can also be applied to higher order half-vortex states.

Ring-vortex solitons and their stabilities in microcavity polariton condensates

A ring-vortex soliton (RVS) is unstable in a uniform microcavity polariton condensate (MPC). We propose a method for generating a stable RVS in an MPC subjected to a ring-shaped defect potential. The density distribution of a RVS with a pump strength is obtained numerically for a given defect size and strength. Then we discover two types of RVSs: soliton with a dark ring and soliton with a gray ring. And the stabilities of RVSs are dependent on the pump and defect strength.

Heat-assisted self-localization of exciton polaritons

Bosonic condensation of microcavity polaritons is accompanied by their relaxation from the ensemble of excited states into a single quantum state. The excess of energy is transferred to the crystal lattice that eventually involves heating of the structure. Creation of the condensate results in the local increase of the temperature which leads to the red shift of the exciton energy providing the mechanism for polariton self-trapping. By employing the driven-dissipative Gross-Pitaevskii model we predict a new type of a stable localized solution supported by the thermally-induced self-trapping in a one-dimensional microcavity structure. The predicted solution is of a sink-type i.e. it is characterized by the presence of converging density currents. We examine the spontaneous formation of these states from the white noise under spatially localized pumping and analyze the criteria for their stability. The collective bosonic polaron state described here may be considered as a toy model for studies of bosonic stars formed due to the self-gravity effect.

Creation and Manipulation of Stable Dark Solitons and Vortices in Microcavity Polariton Condensates.

Solitons and vortices obtain widespread attention in different physical systems as they offer potential use in information storage, processing, and communication. In exciton-polariton condensates in semiconductor microcavities, solitons and vortices can be created optically. However, dark solitons are unstable and vortices cannot be spatially controlled. In the present work we demonstrate the existence of stable dark solitons and vortices under nonresonant incoherent excitation of a polariton condensate with a simple spatially periodic pump. In one dimension, we show that an additional coherent light pulse can be used to create or destroy a dark soliton in a controlled manner. In two dimensions we demonstrate that a coherent light beam can be used to move a vortex to a specific position on the lattice or be set into motion by simply switching the periodic pump structure from two-dimensional (lattice) to one-dimensional (stripes). Our theoretical results open up exciting possibilities for optical on-demand generation and control of dark solitons and vortices in polariton condensates.

Ultra-low threshold polariton condensation.

We demonstrate the condensation of microcavity polaritons with a very sharp threshold occurring at a two orders of magnitude pump intensity lower than previous demonstrations of condensation. The long cavity lifetime and trapping and pumping geometries are crucial to the realization of this low threshold. Polariton condensation, or "polariton lasing" has long been proposed as a promising source of coherent light at a lower threshold than traditional lasing, and these results indicate some considerations for optimizing designs for lower thresholds.

Two-dimensional symbiotic solitons and vortices in binary condensates with attractive cross-species interaction

We consider a two-dimensional (2D) two-component spinor system with cubic attraction between the components and intra-species self-repulsion, which may be realized in atomic Bose-Einstein condensates, as well as in a quasi-equilibrium condensate of microcavity polaritons. Including a 2D spatially periodic potential, which is necessary for the stabilization of the system against the critical collapse, we use detailed numerical calculations and an analytical variational approximation (VA) to predict the existence and stability of several types of 2D symbiotic solitons in the spinor system. Stability ranges are found for symmetric and asymmetric symbiotic fundamental solitons and vortices, including hidden-vorticity (HV) modes, with opposite vorticities in the two components. The VA produces exceptionally accurate predictions for the fundamental solitons and vortices. The fundamental solitons, both symmetric and asymmetric ones, are completely stable, in either case when they exist as gap solitons or regular ones. The symmetric and asymmetric vortices are stable if the inter-component attraction is stronger than the intra-species repulsion, while the HV modes have their stability region in the opposite case.

Nonequilibrium and nonlinear defect states in microcavity-polariton condensates.

The nonequilibrium and nonlinear defect modes (NNDMs), localized by a defect in a nonequilibrium microcavity-polariton condensate (MPC), are studied. There are three analytic solutions of NNDMs in a point defect: the bright NNDM, a bound state with two dark solitons for an attractive potential, and a gray soliton bound by a defect for a repulsive potential. We find that the stable NNDMs in a nonequilibrium MPC are the bright NNDM and gray soliton bound by a defect. The bright NNDM, which has the hyperbolic cotangent form, is a bright localized state existing in a uniform MPC. The bright NNDM is a unique state occurring in a nonequilibrium MPC that has pump-dissipation and repulsive-nonlinearity characters. No such state can exist in an equilibrium system with repulsive nonlinearity.

Vortex and trapped states of microcavity-polariton condensates in a harmonic trap

We investigate the vortex and trapped states of a microcavity-polariton condensate in a harmonic trap under non-resonant excitation. The stationary states of the microcavity-polariton condensate are obtained from the complex Gross–Pitaevskii equation numerically. From the excitations of vortices and trapped states, the boundaries separating stable and unstable states are plotted in the phase diagram shown by pump powers versus pump-spot sizes. Our analysis provides physical parameters for experiments of creating stable vortices and trapped states in a trapped microcavity-polariton condensate.

Stable gray soliton pinned by a defect in a microcavity-polariton condensate.

We study the spatially localized dark state, called dark soliton, in a one-dimensional system of the non-resonantly pumped microcavity-polariton condensate (MPC). From the recent work by Xue and Matuszewski [Phys. Rev. Lett. 112, 216401 (2014)], we know that the dark soliton in the pure MPC system is unstable. But we find that a dark soliton pinned by a defect in the impure MPC becomes a gray soliton and can be stabilized by the presence of a defect. Moreover, the stable regime of the gray soliton is given in terms of the defect strength and pump parameter.

Vortex dynamics of rotating Bose-Einstein condensate of microcavity polaritons

In this work we perform a numerical study of a rotating, harmonically trapped, Bose-Einstein condensate of microcavity polaritons. An efficient numerical method (toolbox) to solve the complex Gross-Pitaevskii equation is developed. Using this method, we investigate how the behavior of the number of vortices formed inside the condensate changes as the various system parameters are varied. In contrast to the atomic condensates, we show, there exists an (experimentally realizable) range of parameter values in which all the vortices can be made to vanish even when there is a high rotation. We further explore how this region can be tuned through other free parameters and also discuss how this study can help to realize the synthetic magnetic field for polaritons and hence paving the way for the realization of the quantum Hall physics and many other exotic phenomena.

Instability of a spontaneously formed multiple-flux vortex in polariton condensates

A spontaneously formed vortex can occur in an incoherently pumped microcavity-polariton condensate which is a non-equilibrium system. The stability of this vortex depends on the flux quanta that the vortex owns. In this paper, we have investigated the dynamics of spontaneously formed vortices with multiple-flux quanta in incoherently pumped microcavity-polariton condensates. From the excitation spectra of a spontaneously formed vortex, we can study the stability of the vortex under different pump strength. The vortices with flux quanta less than four are dynamically stable within a wide range of pump strength. However, when the flux quanta of a vortex is greater than three and the pump power is above a critical pump strength, some imaginary parts of excitation modes of the vortex become positive. The vortex then becomes dynamically unstable and tends to split into several vortices with smaller flux quanta. • Without considering a potential trap or disorders, we study the dynamics of SFMVs in incoherently pumped MPCs through the complex Gross–Pitaevskii equation (cGPE) which is coupled to the reservoir polaritons. • The size of SFMVs is inversely proportional to the square root of the pump power above the threshold. Moreover, the visibility of the vortex goes higher with respective to pump power increasing. • The stability and collective-excitation spectra of a SFMV is investigated by applying the Bogoliubov excitation. When the pump power is low, we find that SFMVs subjected to excitations in an incoherently pumped MPC are surprisingly stable even without stirring, trapping or disorder potentials.

Relaxation Oscillations in the Formation of a Polariton Condensate

We report observation of oscillations in the dynamics of a microcavity polariton condensate formed under pulsed nonresonant excitation. While oscillations in a condensate have always been attributed to Josephson mechanisms due to a chemical potential unbalance, here we show that under some localization conditions of the condensate, they may arise from relaxation oscillations, a pervasive classical dynamics that repeatedly provokes the sudden decay of a reservoir, shutting off relaxation as the reservoir is replenished. Using nonresonant excitation, it is thus possible to obtain condensate injection pulses with a record frequency of 0.1 THz.

Polarized emission in polariton condensates: Switching in a one-dimensional natural trap versus inversion in two dimensions

We perform polarization resolved spectroscopy of two- and one-dimensional microcavity-polariton condensates, which are formed by exciting the system in the optical parametric oscillator configuration. We observe polarization inversion for linearly polarized pumping parallel to the wire in both the 1D and 2D systems. As the polarization plane of the pump is rotated, the degree of linear polarization of the 2D system oscillates between orthogonal polarizations with the same period as that of the pump. However, the 1D system switches abruptly between two states of high degree of linear polarization with half the period. Two complementary models, based on semiclassical Boltzmann kinetic equations and the Gross-Pitaevskii equation, respectively, obtain an excellent agreement with the experimental results, providing a deep insight into the mechanisms responsible for the polarization switching.

Collective excitations, Nambu-Goldstone modes, and instability of inhomogeneous polariton condensates

We study non-equilibrium microcavity-polariton condensates (MPCs) in a harmonic potential trap theoretically. We calculate and analyze the steady state, collective-excitation modes and instability of MPCs. Within excitation modes, there exist Nambu-Goldstone modes that can reveal the pattern of the spontaneous symmetry breaking of MPCs. Bifurcation of the stable and unstable modes is identified in terms of the pumping power and spot size. The unstable mechanism associated with the inward supercurrent flow is characterized by the existence of a supersonic region within the condensate.

Stability and excitations of spontaneous vortices in polariton condensates

We study the dynamics of spontaneously formed vortices in incoherently pumped homogeneous microcavity polariton condensates (MPC). We find vortices are stable and appear spontaneously without stirring or rotating MPCs by the numerical modeling using complex Gross–Pitaevskii equation. The center of the vortex core contains some background of reservoir polaritons and the visibility increases with the pump strength. The vortex radius is inversely proportional to the square root of the condensate density or the pump strength. Finally, vortices formed by low pumping power exhibit short lifetime because of the existence of excitations without costing energy.

Dynamics of the Formation and Decay of Coherence in a Polariton Condensate

We study the dynamics of the formation and decay of a condensate of microcavity polaritons. We investigate the relationship among the number of particles, the emission linewidth, and its degree of linear polarization, which serves as the order parameter. Tracking the condensate formation, we show that coherence is not determined only by occupation of the ground state, bringing new insights into the determining factors for Bose-Einstein condensation.

Going with the flow

Observations of superfluid behaviour — flow without friction — of unusual character in a condensed-matter system pave the way to investigations of superfluidity in systems that are out of thermal equilibrium. It has been predicted that excitons, bound pairs of electrons and holes, should form Bose–Einstein-like condensates at relatively high temperatures. Recently, evidence has been reported of a BEC in a semiconductor microcavity formed by exciton-polaritons, which are a mixture of excitons and photons. Superfluid-like behaviour is expected in such polariton condensates, though it would be of an unusual character as microcavity polaritons are short-lived, so the condensates are not in thermal equilibrium. Amo et al. have developed a pulsed optical excitation technique to set a microcavity polariton condensate in motion and they observe clear manifestations of collective dynamics with superfluid characteristics, such as flow without resistance when crossing a defect. This work points into a new direction to study the dynamics of out-of-equilibrium condensates.

Phase diagram for condensation of microcavity polaritons: From theory to practice

The first realization of a polariton condensate was recently achieved in a CdTe microcavity [Kasprzak et al., Nature (London) 443, 409 (2006)]. We compare the experimental phase boundaries, for various detunings and cryostat temperatures, with those found theoretically from a model which accounts for features of microcavity polaritons such as reduced dimensionality, internal composite structure, disorder in the quantum wells, polariton-polariton interactions, and finite lifetime.