Light carries spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Modern nanooptics, photonics, and plasmonics, tend to explore subwavelength scales and additional degrees of freedom of structured, i.e., spatially-inhomogeneous, optical fields. In such fields, spin and orbital properties become strongly coupled with each other. We overview the fundamental origins and important applications of the main spin-orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at optical interfaces, spin-dependent effects in nonparaxial (focused or scattered) fields, spin-controlled shaping of light using anisotropic structured interfaces (metasurfaces), as well as robust spin-directional coupling via evanescent near fields. We show that spin-orbit interactions are inherent in all basic optical processes, and they play a crucial role at subwavelength scales and structures in modern optics.Light consists of electromagnetic waves that oscillate in time and propagate in space. Scalar waves are described by their intensity and phase distributions. These are spatial (orbital) degrees of freedom, common for all types of waves, either classical or quantum. In particular, propagation of a wave is associated with its phase gradient, i.e., the wavevector or momentum. Importantly, electromagnetic waves are described by vector fields 1 . Therefore, light also possesses intrinsic polarization degrees of freedom, which are associated with the directions of the electric and magnetic fields oscillating in time. In the quantum picture, the right-hand and left-hand circular polarizations, with the electric and magnetic fields rotating about the wavevector direction, correspond to two spin states of photons 2 .Recently there has been enormous interest in spin-orbit interactions (SOI) of light. These are striking optical phenomena where the spin (circular polarization) affects and controls the spatial degrees of freedom of light 3-6 . The intrinsic SOI of light originate from fundamental properties of the Maxwell equations 7,8 and are analogous to the spin-orbit interactions of relativistic quantum particles 2,9,10 and electrons in solids 11,12 . Therefore, fine SOI phenomena appear in all basic optical processes and require revisiting traditional approaches to many optical problems. To mention the most representative examples:(i) A circularly polarized laser beam reflected or refracted at a dielectric interface does not propagate in the original plane but experiences a tiny transverse spin-dependent shift out of this plane. This is a manifestation of the spin-Hall effect of light [13][14][15][16][17][18][19] .(ii) Focusing of circularly-polarized light by a high-numerical-aperture lens results in the generation of a spin-dependent optical vortex (i.e., helical phase producing orbital angular momentum) in the focal field. This is an example of the spin-to-orbital angular momentum conversion in nonparaxial fields [20][21][...

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. While in conventional optics interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near-field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near-field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.Interference is the cornerstone of various phenomena in nature enabling numerous applications. In optics, it is intensively used in microscopy, stellar measurements, spectroscopy, and communication technologies, among many others, and is the basis behind the concepts of reflection, refraction and light bending (1, 2). Typically, interference occurs due to the relative phase lag of different propagating waves. On the other hand, nanophotonics -the branch of optics studying the interaction of light with subwavelength nanoscale structures-deals inherently with phenomena that occur via near-field interactions before appreciable phase lags can be accumulated (3). A radiationless form of interference in the near field (4) is behind new exciting applications such as the focusing of evanescent components to achieve subwavelength resolution in imaging (5-8). Near field interference achieved through the full coherent control of the phase and amplitude of excitation light allows asymmetric spatial field localization (9, 10) and selection of propagation paths at intersections of waveguides (11).We demonstrate near field interference by considering a single source of radiation coupled to a mode with a vectorial structure of electromagnetic field. Using an additional degree of freedom provided by the vectorial character of the field, control over the near-field interference can be achieved. We show that an elliptically polarized dipole can produce destructive or constructive interference of different evanescent components in its near field, and as a result, excite electromagnetic modes in neighbouring material structures, such as dielectric and plasmonic waveguides and diffraction gratings, with a controlled directionality of propagation.

The spin Hall effect leads to the separation of electrons with opposite spins in different directions perpendicular to the electric current flow because of interaction between spin and orbital angular momenta. Similarly, photons with opposite spins (different handedness of circular light polarization) may take different trajectories when interacting with metasurfaces that break spatial inversion symmetry or when the inversion symmetry is broken by the radiation of a dipole near an interface. Here we demonstrate a reciprocal effect of spin-orbit coupling when the direction of propagation of a surface plasmon wave, which intrinsically has unusual transverse spin, determines a scattering direction of spin-carrying photons. This spin-orbit coupling effect is an optical analogue of the spin injection in solid-state spintronic devices (inverse spin Hall effect) and may be important for optical information processing, quantum optical technology and topological surface metrology.

Unidirectional scattering from circularly polarized dipoles has been demonstrated in near-field optics, where the quantum spin-Hall effect of light translates into spin-momentum locking. By considering the whole electromagnetic field, instead of its spin component alone, near-field directionality can be achieved beyond spin-momentum locking. This unveils the existence of the Janus dipole, with side-dependent topologically protected coupling to waveguides, and reveals the near-field directionality of Huygens dipoles, generalizing Kerker's condition. Circular dipoles, together with Huygens and Janus sources, form the complete set of all possible directional dipolar sources in the far- and near-field. This allows the designing of directional emission, scattering, and waveguiding, fundamental for quantum optical technology, integrated nanophotonics, and new metasurface designs.