Spin-orbit interactions of light (1505.02864v2)

Abstract: Light carries spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Modern nano-optics, 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.

• The paper quantifies spin-dependent effects such as the spin-Hall effect and spin-to-orbital angular momentum conversion, highlighting their significance in light propagation.
• It employs theoretical analysis and geometric phase concepts to decompose angular momentum changes during light-matter interactions.
• The study underscores the practical potential of spin-controlled light manipulation in nano-optics, photonics, and next-generation optical devices.

Spin-Orbit Interactions of Light: A Detailed Overview

The paper "Spin-orbit interactions of light" by Bliokh et al. offers a comprehensive examination of the intricate interactions between the spin and orbital angular momentum of light. This work is pivotal for advancing our understanding of light-matter interaction on subwavelength scales, furthering both theoretical insights and practical implementations in fields like nano-optics, photonics, and plasmonics. This essay aims to encapsulate the essence of the paper, focusing on the core phenomena, significant results, and their implications for future research directions.

Spin-orbit interactions (SOI) in optics parallel those found in quantum mechanics, where changes in spin can influence a particle's trajectory. For photons, these interactions manifest across various optical processes, fundamentally altering interpretations within traditional optics frameworks. The authors emphasize that SOI phenomena are not peripheral anomalies but are inherent in every optical interaction, particularly emphasized at subwavelength scales where SOI effects become pronounced.

Key Phenomena of Spin-Orbit Interactions

The paper categorizes SOI effects into several representative phenomena:

1. Spin-Hall Effect (SHE) of Light: Observed in inhomogeneous or anisotropic media, SHE involves spin-dependent shifts in the trajectory of light. This manifests in phenomena like the Imbert-Fedorov shift during beam reflections or refractions at optical interfaces. The authors provide quantitative expressions for these shifts, describing their dependencies on polarization and geometrical constraints.
2. Spin-to-Orbital Angular Momentum Conversion: This occurs notably in nonparaxial fields, such as when circularly polarized light is focused through high-numerical-aperture lenses or during scattering. The conversion emerges from geometrical phases associated with the spatial distribution of wave vectors, impacting the angular momentum characteristics of optical beams.
3. Spin-Directional Coupling: Particularly in evanescent waves, the SOI results in transverse spins that lock their directionality to the propagation path. This intrinsic property, robust across varying setups, has potential applications in developing spin-controlled optical devices and guiding the propagation of guided modes.

Theoretical Underpinnings and Geometric Phases

Central to understanding SOI is the treatment of angular momentum and geometric phases, tailoring the analysis to both fundamental and emergent SOI properties. The angular momentum of light is decomposed into spin angular momentum (SAM) and orbital angular momentum (OAM), each contributing distinctly to observed phenomena. The paper highlights the role of geometric (Berry) phases, which arise from the parallel transport of polarization states across varying wavevector orientations, giving rise to observable spin-dependent shifts and conversions.

Implications and Prospects

The paper transcends purely theoretical examination by addressing the implications of these phenomena in applied optics. SOI effects have vital implications for optical measurement precision and device design. The ability to leverage SOI for spin-controlled light manipulation opens new avenues in photonics, analogous to electron spintronics in solid-state physics. Additionally, the interaction of light with artificial structures such as metasurfaces promises enhanced control over light propagation and interaction, facilitating advanced applications in optical communications and quantum information processing.

Looking forward, the exploration of spin-optical effects in inhomogeneous and structured media promises to expand the toolbox available to physicists and engineers, enhancing control over light behavior at nanoscale dimensions. The inherent robustness and universality of SOI suggest exciting potential for exploitation in practical, next-generation optical systems.

Conclusion

Bliokh et al.’s exploration of the spin-orbit interactions of light significantly enhances our comprehension of light's dynamical characteristics on complex terrains. This paper is a vital contribution to the field, offering insights foundational for both basic research and technological innovation. The exploration of SOI continues to be an essential area of investigation, promising substantive advancements in optical sciences and engineering.