This Review article provides an overview of the fundamental origins and important applications of the main spin–orbit interaction phenomena in modern optics that play a crucial role at subwavelength scales. Light carries both spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Nano-optics, photonics and plasmonics tend to explore subwavelength scales and additional degrees of freedom of structured — that is, spatially inhomogeneous — optical fields. In such fields, spin and orbital properties become strongly coupled with each other. In this Review we cover 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) and robust spin-directional coupling via evanescent near fields. We show that spin–orbit interactions are inherent in all basic optical processes, and that they play a crucial role in modern optics.

Spin-orbit interactions of light

Symmetry-breaking introduced by an adjacent semi-infinite dielectric can introduce coupling and hybridization of the plasmon modes of a metallic nanostructure. This effect is particularly large for entities with a large contact area adjacent to the dielectric. For a nanocube, a nearby dielectric mediates an interaction between bright dipolar and dark quadrupolar modes, resulting in bonding and antibonding hybridized modes. The Fano resonance that dominates the scattering spectrum arises from the interference of these modes. This analysis provides a strategy for optimizing the sensitivity of nanostructures, whether chemically synthesized or grown by deposition methods, as high-performance localized surface plasmon resonance sensors.

/pdf/substrate-induced-fano-resonances-of-a-plasmonic-nanocube-a-50iamdb12n.pdf

Substrate-induced Fano resonances of a plasmonic nanocube: a route to increased-sensitivity localized surface plasmon resonance sensors revealed.

Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/smtd.201600064

Fundamentals and Applications of Metasurfaces

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

/pdf/engineering-metallic-nanostructures-for-plasmonics-and-3c47ox5c4s.pdf

Engineering metallic nanostructures for plasmonics and nanophotonics

A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

/pdf/visible-frequency-metasurface-for-structuring-and-spatially-4kg78hr2wk.pdf

Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices.

The generation of surface plasmon vortices with arbitrary higher order vortex topological charges with novel plasmonic vortex lens is experimentally demonstrated. It is shown that the polarization sensitivity of the plasmonic vortex lens can be utilized for the dynamic switching of the surface plasmon vortices with different topological charges. A simple algebraic rule related to the vortex topological charge change in the dynamic switching is formulated, and its proof is provided with theory and experiment. The synthesis and dynamic switching of higher order surface plasmon vortices have profound potential in optical trapping, optical data storage, and other related fields.

Synthesis and Dynamic Switching of Surface Plasmon Vortices with Plasmonic Vortex Lens

A novel interconnection structure that is capable of transferring surface plasmon energy between two distanced metal-dielectric-metal waveguides is proposed. A numerical analysis indicates that the power transferring efficiency of the proposed structure approaches 88.5% for one millimeter of interconnection length. The main mechanism of the proposed interconnector is governed by Fabry-Perot resonance and a multi-mode interference effect in the junction layers. A physical analysis based on the mechanism for designing an optimized structure is also provided, which could be used to modify the proposed structure.

Highly efficient plasmonic interconnector based on the asymmetric junction between metal-dielectric-metal and dielectric slab waveguides.

A simple and effective optical interconnection which connects two distanced single metal-dielectric interface surface plasmon waveguides by a floating dielectric slab waveguide (slab bridge) is proposed. Transmission characteristics of the suggested structure are numerically studied using rigorous coupled wave analysis, and design rules based on the study are given. In the wave-guiding part, if the slab bridge can support more than the fundamental mode, then the transmission efficiency of the interconnection shows strong periodic dependency on the length of the bridge, due to the multi-mode interference (MMI) effect. Otherwise, only small fluctuation occurs due to the Fabry-Perot effect. In addition, light beating happens when the slab bridge is relatively short. In the wave-coupling part, on the other hand, gap-assisted transmission occurs at each overlapping region as a consequence of mode hybridization. Periodic dependency on the length of the overlap region also appears due to the MMI effect. According to these results, we propose design principles for achieving both high transmission efficiency and stability with respect to the variation of the interconnection distance, and we show how to obtain the transmission efficiency of 68.3% for the 1mm-long interconnection.

Floating dielectric slab optical interconnection between metal-dielectric interface surface plasmon polariton waveguides

We derive mathematical criteria for a pair of guided modes which have the same parity, mutually parallel wave vectors
along the guiding direction of the waveguide, but opposite directions of optical power flow in a negative-index slab
waveguide using a graphical method. It is also proven that the so-called light trap mode corresponds to the degenerate
mode of this pair. We also propose a waveguide structure in which guided light waves can be trapped via tunneling
through thin metamaterial clad layers. This trap is temporary since the trapped light tunnels out completely after a short time.

Minsu Kang

Papers

Synthesis and Dynamic Switching of Surface Plasmon Vortices with Plasmonic Vortex Lens

Highly efficient plasmonic interconnector based on the asymmetric junction between metal-dielectric-metal and dielectric slab waveguides.

Floating dielectric slab optical interconnection between metal-dielectric interface surface plasmon polariton waveguides

Guided modes and light trapping in the negative-refractive-index slab waveguides