Spontaneous Spin Bifurcations and Ferromagnetic Phase Transitions in a Spinor Exciton-Polariton Condensate

Abstract

We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatiallytrapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two ellipticallypolarized (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.Condensation of exciton-polaritons (polaritons) spontaneously breaks the global phase symmetry [1][2][3][4][5]. Owing to their easy optical interrogation, high-speed (ps) interactions, and macroscopic coherence (over hundreds of microns) [6], polariton condensates are excellent candidates to probe and exploit for sensing [7,8], spinoptronics [9][10][11], new optoelectronic devices [12][13][14], and quantum simulators [15]. The driven-dissipative multicomponent polariton system can undergo additional bifurcations and condense into states which are not eigenstates of the single-particle Hamiltonian, but many-body states with reduced symmetry [16,17]. Thus, we should expect that two-component exciton-polariton condensates can also show spontaneous symmetry breaking bifurcations in their polarization state. Spin studies of microcavity polaritons have been of great interest in recent years [18][19][20][21][22][23][24][25][26][27][28][29]. However, spontaneous symmetry-breaking bifurcation of spin has not been observed before.Here, we demonstrate spontaneous magnetization in an exciton-polariton condensate, as a direct result of bifurcations in the spin degree of freedom. Utilizing an optically trapped geometry, condensates spontaneously emerge in either of two discrete spin-polarized states that are stable for many seconds, > 10 10 longer than their formation time. These states emit highly circularly-polarized coherent light (up to 95%) and have opposite circular polarizations. The condensate stochastically condenses in a left-or right-circularly polarized state, with an occurrence likelihood that can be controlled by the ellipticity * ho278@cam.ac.uk † jjb12@cam.ac.uk of the nonresonant pump. The two spin-polarized states can be initialized and switched from one state to another with weak resonant optical pulses. Our system has potential applications in sensing, optical spin memories and spin switches, and it can be implemented for studying long-range spin interactions in polariton condensate lattices. This article is structured as follows: in Section I we review trapped polariton condensates and the current understanding of polarization in untrapped polariton condensates. In Section ...

Spatial multistability induced by cross interactions of confined polariton modes

“…Numerous examples of these interactions have been studied, ranging from polariton condensate interacting with an exciton reservoir in nonresonant excitation experiments [44][45][46][47][48][49] to spinor interactions in resonant [50][51][52][53][54][55] or nonresonant excitation [56,57]. They are, however, different in nature from the spinor effects induced by the effective magnetic field caused by TE-TM splitting of the microcavity mode [58][59][60] or by the birefringence field induced by disorder [61,62].…”

Exciton polaritons in two-dimensional dichalcogenide layers placed in a planar microcavity: Tunable interaction between two Bose-Einstein condensates

“…[34] Recently, spontaneous symmetry-breaking bifurcations in the polarization state of two-component exciton-polariton condensates were demonstrated. [35] Here one can also expect spinor BECs with an interplay between spin-dependent dynamics and Bose-Einstein condensation [5] and distinct Bogolyubov-type elementary excitations, [36] which experimental studies could be performed at much higher temperatures compared to atomic condensates. Yet, exciton-polariton systems with two possible polarization projections onto the growth axis of the hosting semiconductor heterostructure cannot be considered as strictly two-species condensates because of the presence of a spin-flipping exciton-exciton scattering.…”

Persistent circular currents of exciton-polaritons in cylindrical pillar microcavities

“…As a consequence, the newly created condensate, formed once the pump is switched on again, completely loses the information about its previous state. If the persistent circular current had arisen stochastically, as a result of a chaotic uncertainty of the polariton density near the condensation threshold [11,29,30], then, the subsequent switching on of the excitation light would have created interferential spirals of opposite helicities with equal probabilities. However, the experimentally observed spiral conserves its helicity in every switching off and on of the pump.…”