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Spin angular momenta play important roles in light–matter interactions, leading to the emergence of the spin Hall effect and topological quasiparticles in modern optics. The typical approach is to decompose the spins of plane electromagnetic waves into longitudinal and transverse components, yet this description is not easily transferable to more structured electromagnetic environments. Here, we developed a field theory to reveal the physical origin and topological properties of longitudinal and transverse spins for arbitrary electromagnetic waves (including water waves and acoustic waves) in both near-field and free space. For electromagnetic waves carrying intrinsic helicity, we observed the emergence of helicity-dependent transverse spin possessing helicity-dependent spin-momentum locking. To verify that the number of spin-momentum locking states coincides with the spin Chern number, we experimentally measured the three-dimensional spin angular momentum densities of Bloch-type optical skyrmions. Our findings yield valuable insight for constructing spin-based field theory and exploiting optical topological quasiparticle-based applications. The task of mapping solid-state spin-orbit coupling (SOC) into photonic systems has sparked intense research efforts. The authors propose a unified theory to study SOC in photonic and classical wave systems, that is validated numerically and through ad hoc experiments with Bloch-type photonic skyrmions, showing excellent agreement between the two. [ABSTRACT FROM AUTHOR]

Photonic spin skyrmions with deep‐subwavelength features have aroused considerable interest in recent years. However, the manipulation of spin structure in the skyrmions in a desired manner is still a challenge, while this is crucial for developing the skyrmion‐based applications. Here, an approach of optical spin manipulation by utilizing the spin‐momentum equation is proposed to investigate the spin texture in a photonic skyrmion‐pair. With the benefit of the proposed approach, a unique spin texture with spin angular momentum varying linearly along the line connecting the two skyrmion centers is theoretically designed and experimentally verified. The optimized spin texture is then applied in a displacement‐sensing system, which is capable of attaining pico‐metric sensitivity. Compared with the conventional polarization and phase schemes, the spin‐based manipulation mechanism provides a new pathway for optical modulation, which is of great value in nanophotonics from both fundamental and application. [ABSTRACT FROM AUTHOR]

Hyperbolic metamaterials can support extremely high‐k wavevectors, also known as volume plasmon polaritons, with highly directional propagation and light confinement to deep‐subwavelength scales. The number of high‐k modes has an inverse relationship with the overall thickness of hyperbolic metamaterials. Here, the optical spin textures from high‐k mode polaritons on the surface of silica–silver multilayer hyperbolic metamaterials are studied. The results demonstrate that with an increase in the overall thickness, the spin texture changes from a twisted structure to a Néel‐type photonic skyrmion, due to the decrease in the number of high‐k modes. Moreover, the size of a single photonic skyrmion is less than 1/2 of the light wavelength, and the full width at half maximum of the spin angular momentum is less than 1/9 of the light wavelength. This work is expected to facilitate new applications of hyperbolic metamaterials, such as spin‐dependent excitation and detection devices at the deep‐subwavelength level. [ABSTRACT FROM AUTHOR]

Spin angular momentum associated with circular polarization is a fundamental and important aspect of photons both in classical and quantum optics. The interaction of this optical spin with matter and structures results in many intriguing optical effects and state-of-the-art applications covered under the emerging subject of spin optics. Distinct from longitudinal optical spin along the mean wavevector, transverse spin, the corresponding vector of which is perpendicular to the mean wavevector, prevails and plays a significant role in confined electromagnetic waves such as focused beams, guided waves, and evanescent waves. In the optical near-field, these transverse spins are generated owing to the spatial variation of the kinetic momentum of confined electromagnetic waves, where the spin and orbital angular momenta are strongly coupled, leading to many interesting topological spin structures and properties. Several reviews on optical transverse spins have been published in recent years in which their concepts and the various configurations producing them were introduced systematically. Here, we introduce in this review the underlying physics and dynamics of transverse spin and the resultant topological structures and properties such as the photonic skyrmions and merons. We term this sub-area 'spin photonics', its scope being to cover the design and research of spin structures in strongly confined electromagnetic fields with unique properties and applications. The concepts and framework reviewed have importance in optics, topological photonics, metrology, and quantum technologies and may be used to extend spin-dynamics concepts to fluidic, acoustic, and gravitational waves. [ABSTRACT FROM AUTHOR]

We proposed and built a near-field scanning optical microscope (NSOM) to enable the characterization of the spin angular momentum (SAM) distribution of electromagnetic fields with nanoscale resolution. The NSOM probe was composed of a circular nanohole formed in a thick gold film that was deposited on a tapered cone fiber. The near-field signal, when coupled through the nanohole to the fiber, was split and analyzed using a combination of a quarter-wave plate and a polarizer to extract the two circular polarization components of the signal. This allowed us to characterize the out-of-plane SAM component, which was determined using the relationship Sz ∝ IRCP − ILCP. Using the developed system, we mapped the SAM distributions of a variety of tightly focused cylindrical vector vortex beams and thus validated the system's effectiveness. The proposed spin-resolved NSOM could be a valuable tool for studies of both near-field spin optics and topological photonics. [ABSTRACT FROM AUTHOR]

Traditional detour-phase hologram is a powerful optical device for manipulating phase and amplitude of light, but it is usually not sensitive to the polarization of light. By introducing the light-metasurface interaction mechanism to the traditional detour phase hologram, we design a novel plasmonic nano-slits assisted polarization selective detour phase meta-hologram, which has attractive advantages of polarization multiplexing ability, broadband response, and ultra-compact size. The meta-hologram relies on the dislocations of plasmonic slits to achieve arbitrary phase distributions, showing strong polarization selectivity to incident light due to the plasmonic response of deep-subwavelength slits. To verify its polarization sensitive and broadband responses, we experimentally demonstrate two holographic patterns of an optical vortex and an Airy beam at p- and s-polarized light with wavelengths of 532nm, 633nm and 780nm, respectively. Furthermore, we realize an application example of the meta-hologram as a polarization multiplexed photonic device for multi-channel optical angular momentum (OAM) generation and detection. Such meta-holograms could find widespread applications in photonics, such as chip-level beam shaping and high-capacity OAM communication. [ABSTRACT FROM AUTHOR]

Since the seminal work by J. H. Poynting, light has been known to carry momentum and angular momentum. The typical dynamical features of light and its interactions—termed spin–orbit interactions (SOIs), which have been investigated intensely over the last 30 years—play a crucial role in various light-matter interactions, for example: spin Hall effect, spin–orbit conversion, helicity-controlled unidirectional excitation of light, and their inverse effects, which leads to plenty of applications including optical manipulation, communications, imaging, sensing, nanometrology, on-chip optoelectronic technologies and interdisciplinary researches. In particular, the SOI of light in isotropic inhomogeneous media is a fine, subwavelength effect accomplished through the intrinsic coupling between light's phase, polarization and position. Therefore, the traditional methods of near-field measurements, such as near field scanning optical microscopy (NSOM), have been widely employed to reveal the optical SOIs intuitively by measuring the intensity of light. Very recently, with modern advanced nanofabrication techniques, many measurement techniques based on nanoparticles, nanoantennas, and nanoprobes of special designs have been proposed to understand the optical SOIs visually by characterizing the polarization and spin/orbital features of light. This endeavor has led to the development of chiral quantum optics, spin optics, and topological photonics, and resulted in novel applications requiring optical manipulations and angular momentum communications, chiral imaging, nanometrology, and robust spin-based devices and techniques for quantum technologies. Here, we review the near-field techniques for measurements of optical SOIs together with their potential applications. We start with a theoretical overview of momentum and angular momentum properties of generic optical fields and typical phenomena involving optical SOIs. Then, we overview the theoretical basis and latest achievements of the near-field measurement techniques, including NSOM, optical manipulations, nanoantenna, and nanoprobes of special designs, all relevant to optical SOIs. A comprehensive classification is then constructed of all known methods of optical near-field measurements for the SOI of light and novel techniques identified for future applications. [ABSTRACT FROM AUTHOR]