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.

We report on the first experimental observation of discrete vortex solitons in two-dimensional optically-induced photonic lattices. We demonstrate strong stabilization of an optical vortex by the lattice in a self-focusing nonlinear medium and study the generation of the discrete vortices from a broad class of singular beams.PACS numbers: 42.65. Tg, 42.65.Jx, 42.70.Qs Periodic photonic structures and photonic crystals recently attracted a lot of interest due to the unique ways they offer for controlling light propagation. Periodic modulation of the refractive index modifies the diffraction properties and strongly affects nonlinear propagation and localization of light [1]. Recently, many nonlinear effects including the formation of lattice solitons have been demonstrated experimentally in one-and twodimensional optically-induced photonic lattices [2,3,4,5]. The concept of optically-induced lattices [6] relies on the modulation of the refractive index of a nonlinear medium with periodic optical patterns, and the use of a weaker probe beam to study scattering of light from the resulting periodic photonic structure.So far, only simple stationary structures have been described theoretically and generated experimentally in optically-induced lattices [2,3,4,5,6]. One of the most important next steps is the study of nonlinear modes with a nontrivial phase such as vortices, the fundamental localized objects appearing in many branches of physics. In optics, vortices are associated with the screw phase dislocations carried by diffracting optical beams [7]. When such vortices propagate in a defocusing nonlinear Kerrlike medium, the vortex core becomes self-trapped, and the resulting structure is known as an optical vortex soliton [1]. Such vortex solitons are usually generated experimentally on a broad background beam [8,9]. They demonstrate many similarities with the vortices observed in superfluids and Bose-Einstein condensates.In contrast, optical vortex solitons do not exist in a self-focusing nonlinear medium; a ring-like optical beam with a phase dislocation carrying a finite orbital angular momentum [10] decays into the fundamental solitons flying off the main ring [11]. This effect was first observed experimentally in saturable Kerr-like nonlinear medium [12], and then in photorefractive [9] and quadratic [13] nonlinear media in the self-focusing regime.Recent theoretical studies of the discrete [14] and continuous models of nonlinear periodic lattices [15,16] suggest that the vortex-like structures can be supported by the lattice even in the self-focusing regime. In this Letter, we report on the first experimental observation of discrete (lattice) vortex solitons and demonstrate, both theoretically and experimentally, that localized optical vortices can be generated in a self-focusing nonlinear medium, being stabilized by the two-dimensional periodic potential of a photonic lattice.To lay a background for our experiment, first we study numerically the generation of discrete vortex solitons in a two-dimensional ...

We give an exact self-consistent operator description of the spin and orbital angular momenta, position, and spin-orbit interactions of nonparaxial light in free space. Both quantum-operator formalism and classical energy-flow approach are presented. We apply the general theory to symmetric and asymmetric Bessel beams exhibiting spin-and orbital-dependent intensity profiles. The exact wave solutions are clearly interpreted in terms of the Berry phases, quantization of caustics, and Hall effects of light, which can be readily observed experimentally

We study experimentally nonlinear localization effects in optically induced gratings created by interfering plane waves in a photorefractive crystal. We demonstrate the generation of spatial bright solitons similar to those observed in arrays of coupled optical waveguides. We also create pairs of out-of-phase solitons, which resemble twisted localized states in nonlinear lattices.

We show, both theoretically and experimentally, that high-numerical-aperture (NA) optical microscopy is accompanied by strong spin-orbit interaction of light, which translates fine information about the specimen to the polarization degrees of freedom of light. An 80 nm gold nanoparticle scattering the light in the focus of a high-NA objective generates angular momentum conversion, which is seen as a nonuniform polarization distribution at the exit pupil. We demonstrate remarkable sensitivity of the effect to the position of the nanoparticle: Its subwavelength displacement produces the giant spin-Hall effect, i.e., macroseparation of spins in the outgoing light. This brings forth a far-field optical nanoprobing technique based on the spin-orbit interaction of light.