Exciton-polariton Condensates Research Articles

Relaxation Oscillations of an Exciton-Polariton Condensate Driven by Parametric Scattering.

We report the observation of coherent oscillations in the relaxation dynamics of an exciton-polariton condensate that were driven by parametric scattering processes. As a result of the interbranch scattering scheme and the nonlinear polariton-polariton interactions, such parametric scatterings exhibit a high scattering efficiency that leads to the fast depletion of the polariton condensate and the periodic shut-off of the bosonic stimulation processes, eventually causing relaxation oscillations. Employing polariton-reservoir interactions, the oscillation dynamics in the time domain can be projected onto the energy space. In theory, our simulations using the open-dissipative Gross-Pitaevskii equation are in excellent agreement with experimental observations. Surprisingly, the oscillation patterns, including many excitation pulses, are clearly visible in our time-integrated images, implying the high stability of the relaxation oscillations driven by polariton parametric scatterings.

Stability and Controllability of Various Spatial Solitons in Exciton–Polariton Condensates by a Composite Pumping

We investigate the stability and controllability of one-dimensional bright and dark solitons, and two-dimensional bright solitons and vortices with the charges m = 1 and 2, respectively, in a nonresonantly incoherent pumped exciton–polariton condensates. A composite pumping, consisting of the constant part and the Bessel-type spatially modulating part, is introduced to balance the gain and loss. We demonstrate that the pumping can not only stabilize all these solitons but also modulate the profiles of these solitons. We also find that all these solitons obtained in this study are different from the ones in the previous studies. Our work may pave a way to modulate these solitons in the nonresonantly pumped exciton–polariton system.

Thermalization of Fluorescent Protein Exciton-Polaritons at Room Temperature.

Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultralow threshold room temperature exciton-polariton condensation and lasing. This contribution investigates the thermalization of FP microcavity exciton-polaritons upon optical pumping under ambient conditions. Polariton cooling is realized using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry-Pérot cavity. Interestingly, at the threshold excitation energy (fluence) of ≈9 nJ per pulse (15.6 mJ cm-2 ), an effective temperature is observed, Teff ≈ 350 ± 35 K close to the lattice temperature indicative of strongly thermalized exciton-polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavity Q-factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell-Boltzmann statistics at k‖ = 0 above the threshold. Thermalized polariton gases in organic systems at equilibrium hold substantial promise for designing room temperature polaritonic circuits, switches, and lattices for analog simulation.

Condensation of 2D exciton-polaritons in an open-access microcavity

We establish a tunable open-access microcavity consisting of two planar distributed Bragg reflectors (DBRs) individually controlled by nanopositioners. By varying the cavity length, such configuration enables variation of the light–matter coupling strength by a factor of 2, while keeping in microresonators the same active region and cavity mirrors. Polariton condensation was demonstrated over a large range of Rabi splittings and the corresponding threshold diagram was derived as a function of cavity-exciton detuning, which fits well with theoretical simulations. The results show that for various light-matter coupling strengths, optimal detunings featured by the lowest condensation threshold always occur at a fixed depth of energy trap between the exciton reservoir and the polariton ground state, which enables the most efficient exciton–exciton scattering into the condensate state in the driven-dissipative polaritonic system.

Persistent current by a static non-Hermitian ratchet

We propose a scheme to generate a persistent current in driven-dissipative systems which can be described by the generalized Gross-Pitaevskii (GP) equation. Our proposal consists of fabricating a rachet-potential shape of the loss-rate profile, which simultaneously breaks the time-reversal symmetry and the parity-inversion symmetry. Unlike existing schemes to generate a current using a rachet potential in Hermitian systems, no dynamic drive is needed. The basic physics of our scheme is discussed by a simple discrete driven-dissipative GP model, and the results are also verified by a realistic continuous model. Furthermore, we demonstrate the experimental feasibility of our scheme to generate the persistent current in exciton-polariton condensates in a semiconductor microcavity.

Solving the Max-3-Cut Problem with Coherent Networks

Many computational problems are intractable through classical computing and, as Moore's law is drawing to a halt, demand for finding alternative methods in tackling these problems is growing. Here, we realize a liquid light machine for the NP-hard max-3-cut problem based on a network of synchronized exciton-polariton condensates. We overcome the binary limitation of the decision variables in Ising machines using the continuous-phase degrees of freedom of a coherent network of polariton condensates. The condensate network dynamical transients provide optically-fast annealing of the XY Hamiltonian. We apply the Goemans and Williamson random hyperplane technique, discretizing the XY ground state spin configuration to serve as ternary decision variables for an approximate optimal solution to the max-3-cut problem. Applications of the presented coherent network are investigated in image-segmentation tasks and in circuit design.

Engineering Photon Statistics in a Spinor Polariton Condensate.

We implement full polarization tomography on photon correlations in a spinor exciton-polariton condensate. Our measurements reveal condensate pseudospin mean-field dynamics spanning from stochastic switching between linear polarization components, limit cycles, and stable fixed points, and their intrinsic relation to the condensate photon statistics. We optically harness the cavity birefringence, polariton interactions, and the optical orientation of an incoherent exciton reservoir to engineer photon statistics with precise control. Our results demonstrate a smooth transition from a highly coherent to a super-thermal state of the condensate polarization components.

Localization of anisotropic exciton polariton condensates in perovskite microcavities

Perovskite materials such as CsPbBr3 attract intensive attention due to excellent photonic properties. Especially, the exciton polariton condensate based on perovskite CsPbBr3 has been realized at room temperature. In this work, we observe the XY splitting of exciton polaritons localized in disorder induced potential traps in a CsPbBr3 microcavity due to the anisotropy of the perovskite and polariton condensation onto the horizontally and vertically linear polarized modes. In addition, the refractive index of the microcavity can be modified by filling liquid crystal molecules into the cavity layer, which can modify the XY splitting of localized polaritons. Our results pave the way to study the nontrivial band structure based on the XY splitting of the localized polariton condensate in microcavities.

Stable solitons and their controllability in <inline-formula><tex-math id="M2">\begin{document}$ {\cal{PT}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M2.png"/></alternatives></inline-formula> symmetrical polariton condensates

By constructing the spatial distribution of external potential and incoherent pumping, a <inline-formula><tex-math id="M8">\begin{document}$ {\cal{PT}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M8.png"/></alternatives></inline-formula> symmetrical model satisfied by the one-dimensional incoherent pumped exciton-polariton condensate system is designed. In the weakly nonlinear case, the <inline-formula><tex-math id="M9">\begin{document}$ {\cal{PT}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M9.png"/></alternatives></inline-formula> symmetrical phase transition point is found, and the linear spectrum is shown. In the normal nonlinear case, found are the bright soliton with the zero background, the multi-poles dark solitons with zero background, the symmetry breaking dark solitons and symmetrical dark soliton with the homogeneous background, and the dip- and hump-type dark solitons with the homogeneous background, and discussed are the effects of inhomogeneous pumping and the imaginary part of external potential on the profiles and the stability of solitons. Through these results, the competition between <inline-formula><tex-math id="M10">\begin{document}$ {\cal{PT}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M10.png"/></alternatives></inline-formula> symmetrical potential and the inhomogeneous pumping is understood, the scheme that how the bright and dark solitons are excited is presented, and the existence and stability regions of these solitons are determined. Finally, the symmetry breaking dark solitons are controlled by modulating the imaginary part of the <inline-formula><tex-math id="M11">\begin{document}$ {\cal{PT}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20220475_M11.png"/></alternatives></inline-formula> symmetrical potential, which reveals the potential applications of the polariton condensate system in optical information processing, such as the all-optical switches.

Reservoir optics with exciton-polariton condensates

We investigate an all-optical microscale planar lensing technique based on coherent fluids of semiconductor cavity exciton-polariton condensates. Our theoretical analysis underpins the potential in using state-of-the-art spatial light modulation of nonresonant excitation beams to guide and focus polariton condensates away from their pumping region. The nonresonant excitation profile generates an excitonic reservoir that blueshifts the polariton mode and provides gain, which can be spatially tailored into lens shapes at the microscale to refract condensate waves. We propose several different avenues in controlling the condensate fluid, and demonstrate formation of highly enhanced and localized condensates away from the pumped reservoirs. This opens new perspectives in guiding quantum fluids of light and generating polariton condensates that are removed from detrimental reservoir dephasing effects.

Non-Hermitian spectrum and multistability in exciton-polariton condensates

We investigate non-Hermitian spectrum and multistability of exciton-polariton condensates excited by a nonresonant pump. An increase in pumping power moves the system away from non-Hermitian spectrum degeneracy toward bifurcation through an exceptional point, which induces a transition from monostability to multistability. The spectrum can be used as a universal method for detecting this transition. In the region of multistability, the system contains one steady and two metastable states. The analyses of stability show that metastable states maintain a finite lifetime and eventually evolve into steady states, which are induced by non-Hermitian skin effects. We also discover a steady state with multipeak soliton for attractive polariton-polariton interaction, which is different from general single-peak soliton for repulsive interaction. Our results open up exciting possibilities for controlling non-Hermitian quantum multistable states, which may be useful for designing polariton-based devices exploiting optical multistability.

Half-skyrmions with higher topological quantum numbers in homogeneous exciton-polariton condensates.

We investigate the topological excitations of half-quantum vortices (HQVs) with higher topological quantum numbers in a homogeneous spinor exciton-polariton condensate pumped by a laser beam and an additional coherent light carrying orbital angular momentum. The spin texture and integrated topological charge can be controlled through the pump. Among these textures, the polaritonic half-skyrmions (or polaritonic merons) can be created with a suitable excitation condition. Moreover, when the pump polarization is in favor of the vortex component of the HQV, there is an inversion of circular polarization (spin flipping) from the center of the HQV towards the edge. The radial flipping position can be manipulated by the pump polarization or power. Finally, we demonstrate that the HQVs can stably exist from the linear stability analysis.

Exciton-polariton properties of hexagonal BN-based microcavity and their potential applications in BEC and superconductivity

Microcavity exciton-polaritons are two-dimensional bosonic quasiparticles composed by excitons and photons. Using a model Hamiltonian with parameters generated from ab initio density-functional theory and Bethe-Salpeter equation calculations, we investigate the exciton and the exciton-polariton properties of hexagnonal boron nitride (hBN) based microcavity. We show that hBN-based microcavities, including monolayer and all-dielectric ones, are promising in optoelectronic applications. Room temperature exciton-polariton Bose-Einstein condensation can be achieved because of the large oscillator strength and binding energy of the exciton, and the strong interaction between the exciton-polaritons and the longitudinal optical phonons. Based on this BEC state, an exciton-polariton mediated superconducting device can also be fabricated at a few tens of kelvin using the microcavity structure proposed by Laussy et al. Phys. Rev. Lett. 104, 106402 (2010).

Spontaneous symmetry breaking in persistent currents of spinor polaritons

We predict the spontaneous symmetry breaking in a spinor Bose–Einstein condensate of exciton-polaritons (polaritons) caused by the coupling of its spin and orbital degrees of freedom. We study a polariton condensate trapped in a ring-shaped effective potential with a broken rotational symmetry. We propose a realistic scheme of generating controllable spinor azimuthal persistent currents of polaritons in the trap under the continuous wave optical pump. We propose a new type of half-quantum circulating states in a spinor system characterized by azimuthal currents in both circular polarizations and a vortex in only one of the polarizations. The spontaneous symmetry breaking in the spinor polariton condensate that consists in the switching from co-winding to opposite-winding currents in opposite spin states is revealed. It is characterized by the change of the average orbital angular momentum of the condensate from zero to non-zero values. The radial displacement of the pump spot and the polarization of the pump act as the control parameters. The considered system exhibits a fundamental similarity to a superconducting flux qubit, which makes it highly promising for applications in quantum computing.

Nonreciprocal exciton-polariton ring lattices

Recent experiments have shown the transfer of orbital angular momentum (OAM) from a non-resonant laser onto an exciton-polariton condensate, despite earlier views that the phase information of such a laser should be lost during the process of polariton condensation. We study with a phenomenological theory the interplay of a usual angular momentum independent gain and an angular momentum preserving gain. We find that even when the latter is much smaller, it is enough to favour condensation into a given orbital angular momentum state. This further allows a breaking of symmetry in the system, which further manifests in non-reciprocal one-way propagation in a lattice of coupled rings. Even though we consider only Hermitian reciprocal coupling between rings, the local non-Hermiticity generates an effective non-reciprocal coupling and supports a non-Hermitian topological invariant (winding number) associated to a non-Hermitian skin effect.

Synchronization in disordered oscillator lattices: Nonequilibrium phase transition for driven-dissipative bosons

We show that a lattice of phase oscillators with random natural frequencies, described by a generalization of the nearest-neighbor Kuramoto model with an additional cosine coupling term, undergoes a phase transition from a desynchronized to a synchronized state. This model may be derived from the complex Ginzburg-Landau equations describing a disordered lattice of driven-dissipative Bose-Einstein condensates of exciton polaritons. We derive phase diagrams that classify the desynchronized and synchronized states that exist in both one and two dimensions. This is achieved by outlining the connection of the oscillator model to the quantum description of localization of a particle in a random potential through a mapping to a modified Kardar-Parisi-Zhang equation. Our results indicate that long-range order in polariton condensates, and other systems of coupled oscillators, is not destroyed by randomness in their natural frequencies.

Collective Excitations of Exciton-Polariton Condensates in a Synthetic Gauge Field.

Collective (elementary) excitations of quantum bosonic condensates, including condensates of exciton polaritons in semiconductor microcavities, are a sensitive probe of interparticle interactions. In anisotropic microcavities with momentum-dependent transverse-electric-transverse-magnetic splitting of the optical modes, the excitations' dispersions are predicted to be strongly anisotropic, which is a consequence of the synthetic magnetic gauge field of the cavity, as well as the interplay between different interaction strengths for polaritons in the singlet and triplet spin configurations. Here, by directly measuring the dispersion of the collective excitations in a high-density optically trapped exciton-polariton condensate, we observe excellent agreement with the theoretical predictions for spinor polariton excitations. We extract the interaction constants for polaritons of the same and opposite spin and map out the characteristic spin textures in an interacting spinor condensate of exciton polaritons.

Kagome Flatbands for Coherent Exciton-Polariton Lasing

Kagome lattices supporting Dirac cone and flatband dispersions are well known as a highly frustrated, two-dimensional lattice system. Particularly the flatbands therein are attracting continuous interest based on their link to topological order, correlations and frustration. In this work, we realize coupled microcavity implementations of Kagome lattices hosting exciton-polariton quantum fluids of light. We demonstrate precise control over the dispersiveness of the flatband as well as selective condensation of exciton-polaritons into the flatband. Subsequently, we focus on the spatial and temporal coherence properties of the laser-like emission from these polariton condensates that are closely connected to the flatband nature of the system. Notably, we find a drastic increase in coherence time due to the localization of flatband condensates. Our work illustrates the outstanding suitability of the exciton-polariton system for detailed studies of flatband states as a platform for microlaser arrays in compact localized states, including strong interactions, topology and non-linearity.

Dissipative Josephson vortices in annular polariton fluids

We consider two concentric rings formed by bosonic condensates of exciton-polaritons. A circular superfluid flow of polaritons in one of the rings can be manipulated by acting upon the second annular polariton condensate. The complex coupling between the rings with different topological charges triggers nucleation of stable Josephson vortices (JVs) which are revealed as topological defects of the angular dependence of the relative phase between rings. Being dependent on the coupling strength, the structure of the JV governs the difference of the mean angular momenta of the inner and the outer rings. At the vanishing coupling the condensates rotate independently demonstrating no correlations of their winding numbers. At the moderate coupling, the interaction between two condensates tends to equalize their mean angular momenta despite of the mismatch of the winding numbers demonstrating the phenomenology of a drag effect. Above the critical coupling strength the synchronous rotation is established via the phase slip events.

First-order dissipative phase transition in an exciton-polariton condensate

We investigate the phase diagram of a two-dimensional driven-dissipative system of polaritons coupled to the excitonic reservoir. We find that two critical points exists. The first corresponds to the quasi-condensation and the second to a first-order phase transition from the non-uniform state with spatially modulated density to a uniform state. The latter is related to the modulational instability of a homogeneous state due to the repulsive interactions with the non-condensed reservoir. The first-order character of the transition is evidenced by a discontinuity in the density and the correlation length as well as the phase coexistence and metastability. Moreover, we show that a signature of a Berezinskii-Kosterlitz-Thouless-like transition can be observed in the non-uniform phase.