The spin Hall effect (SHE) of light, as an analogue of the SHE in electronic systems, is a promising candidate for investigating the SHE in semiconductor spintronics/valleytronics, high-energy physics and condensed matter physics, owing to their similar topological nature in the spin–orbit interaction. The SHE of light exhibits unique potential for exploring the physical properties of nanostructures, such as determining the optical thickness, and the material properties of metallic and magnetic thin films and even atomically thin two-dimensional materials. More importantly, it opens a possible pathway for controlling the spin states of photons and developing next-generation photonic spin Hall devices as a fundamental constituent of the emerging spinoptics. In this review, based on the viewpoint of the geometric phase gradient, we give a detailed presentation of the recent advances in the SHE of light and its applications in precision metrology and future spin-based photonics.

The spin Hall effect (SHE) of light is a useful metrological tool for characterizing the structure parameters' variations of nanostructure. In this letter, we propose using the SHE of light to identify the graphene layers. This technique is based on the mechanism that the transverse displacements in SHE of light are sensitive to the variations of graphene layer numbers.

The photonic spin Hall effect (SHE) in the reflection and refraction at an interface is very weak because of the weak spin-orbit interaction. Here, we report the observation of a giant photonic SHE in a dielectric-based metamaterial. The metamaterial is structured to create a coordinate-dependent, geometric Pancharatnam–Berry phase that results in an SHE with a spin-dependent splitting in momentum space. It is unlike the SHE that occurs in real space in the reflection and refraction at an interface, which results from the momentum-dependent gradient of the geometric Rytov–Vladimirskii–Berry phase. We theorize a unified description of the photonic SHE based on the two types of geometric phase gradient, and we experimentally measure the giant spin-dependent shift of the beam centroid produced by the metamaterial at a visible wavelength. Our results suggest that the structured metamaterial offers a potential method of manipulating spin-polarized photons and the orbital angular momentum of light and thus enables applications in spin-controlled nanophotonics.

In this work, we develop a hybrid-order Poincaré sphere to describe the evolution of polarization states of wave propagation in inhomogeneous anisotropic media. We extend the orbital Poincaré sphere and high-order Poincaré sphere to a more general form. Polarization evolution in inhomogeneous anisotropic media with special geometry can be conveniently described by state evolution along the longitude line on the hybrid-order Poincaré sphere. Similar to that in previously proposed Poincaré spheres, the Berry curvature can be regarded as an effective magnetic field with monopole centered at the origin of sphere and the Berry connection can be interpreted as the vector potential. Both the Berry curvature and the Pancharatnam-Berry phase on the hybrid-order Poincaré sphere are demonstrated to be proportional to the variation of total angular momentum. Our scheme provides a convenient method to describe the spin-orbit interaction in inhomogeneous anisotropic media.

Vortex beams have been extensively realized using different approaches. Typically, the efficiency and bandwidth of a vortex beam are limited by impure copolarized components and the intrinsic dispersion of passive resonant structures. Here, we propose a strategy to generate wideband vortex beams by using a Pancharatnam–Berry metasurface in which two orthogonal reflections exhibit a broadband out-of-phase difference. To achieve this, a broadband strategy based on multimode operation and dispersion engineering methods was established. A dual-layer meta-atom is proposed; each layer comprises of five metallic dipoles, and the geometrical parameters are carefully adjusted to tune the resonant frequencies. Because the dipole orientations in each layer are orthogonal, the reflection responses under the two orthogonal polarizations can be independently engineered. Both numerical and experimental results indicate that our method not only enables a high-efficiency spiral beam conversion over a broad range of 6.95–18 GHz (>82%) but also causes a polarization-insensitive effect; thus, it can be adapted for any linear or circular polarization.