Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.

Controlled transfer of orbital angular momentum to an exciton-polariton Bose-Einstein condensate spontaneously created under incoherent, off resonant excitation conditions is a long-standing challenge in the field of microcavity polaritonics. We demonstrate, experimentally and theoretically, a simple and efficient approach to the generation of nontrivial orbital angular momentum states by using optically induced potentials-chiral polaritonic lenses. These lenses are produced by a structured optical pump with a spatial distribution of intensity that breaks the chiral symmetry of the system.

The semiconductor polariton laser promises a new source of coherent light, which, compared to conventional semiconductor photon lasers, has input-energy threshold orders of magnitude lower. However, intensity stability, a defining feature of a coherent state, has remained poor. Intensity noise at many times of the shot-noise of a coherent state has persisted, which has been attributed to multiple mechanisms that are difficult to separate in conventional polariton systems. The large intensity noise in turn limited the phase coherence. These limit the capability of the polariton laser as a source of coherence light. Here, we demonstrate a polariton laser with shot-noise limited intensity stability, as expected of a fully coherent state. This is achieved by using an optical cavity with high mode selectivity to enforce single-mode lasing, suppress condensate depletion, and establish gain saturation. The absence of spurious intensity fluctuations moreover enabled measurement of a transition from exponential to Gaussian decay of the phase coherence of the polariton laser. It suggests large self-interaction energies in the polariton condensate, exceeding the laser bandwidth.Such strong interactions are unique to matter-wave laser and important for nonlinear polariton devices. The results will guide future development of polariton lasers and nonlinear polariton devices.

Semiconductor exciton-polaritons in planar microcavities form coherent two-dimensional condensates in non-equilibrium. However, the coupling of multiple lower-dimensional polariton quantum systems, which are critical for polaritonic quantum device applications and novel cavity-lattice physics, has been limited in conventional cavity structures. Here, we demonstrate full non-destructive confinement of polaritons using a hybrid cavity composed of a single-layer subwavelength grating mirror and a distributed Bragg reflector. Single-mode polariton lasing was observed at a chosen polarization. The incorporation of a designable slab mirror in a conventional vertical cavity, when operating in the strong-coupling regime, enables the confinement, control and coupling of polariton gasses in a scalable fashion. This approach may open the door to experimental implementations of polariton-based quantum photonic devices and coupled cavity quantum electrodynamic systems. 2-4 Exciton-polaritons are formed via strong coupling between excitons and photons. Due to the excitonic component, the polaritons are massive, weakly interacting quasiparticles that feature strong nonlinearity and rich many-body physics.5 By mixing with the photon, polaritons have an effective mass that is 10 28 of the hydrogen atomic mass, and they are relatively insensitive to disorder or localization potentials in the active media. Hence, polaritons exhibit quantum coherence over macroscopic scales with high critical temperatures. Polaritons in quantum-well microcavities couple out of the cavity at a fixed rate while conserving the energy and in-plane wavenumber, providing direct experimental access that is unavailable in typical many-body quantum systems. Hallmarks of non-equilibrium condensation and superfluidity have been widely observed in isolated two-dimensional (2D) polariton systems (Ref. 3 and the references therein).Foundational work on 2D polariton systems has inspired theoretical schemes for polariton-based quantum circuits, 6-8 quantum light sources 9-12 and novel quantum phases. 4 Experimental implementation of these schemes requires the control, confinement and coupling of polariton systems, which remain challenging in conventional microcavity structures. Important features of a versatile experimental platform based on polaritons include: first, well-defined zero-dimensional (0D) polaritons as building blocks of a coupled system; second,

We observe a strong variation of the Zeeman splitting of exciton polaritons in microcavities when switching between the linear regime, the polariton lasing, and photon lasing regimes. In the polariton lasing regime the sign of Zeeman splitting changes compared to the linear regime, while in the photon lasing regime the splitting vanishes. We additionally observe an increase of the diamagnetic shift in the polariton lasing regime. These effects are explained in terms of the nonequilibrium "spin Meissner effect."