Transverse spin dynamics of light: the generalized spin-momentum locking for structured guided modes

TL;DR

This paper extends the quantum spin-Hall effect of light to structured guided modes, revealing a generalized spin-momentum locking that governs the vectorial properties of electromagnetic waves, with implications for spin optics and topological photonics.

Contribution

It introduces a generalized spin-momentum relationship for vectorial guided waves, demonstrating out-of-plane transverse spins and their lock to energy flow, both theoretically and experimentally.

Findings

Out-of-plane transverse optical spins vary with energy flow direction.

Demonstration of spin-momentum locking in structured guided waves.

Experimental validation with four types of surface structured waves.

Abstract

Quantum spin-Hall effect, a manifestation of topological properties that govern the behavior of surface states, was studied intensively in condensed matter physics resulting in the discovery of topological insulators. The quantum spin-Hall effect of light was introduced for surface plane-waves which intrinsically carry transverse optical spin, leading to many intriguing phenomena and applications in unidirectional waveguiding, metrology and quantum technologies. In addition to spin, optical waves can exhibit complex topological properties of vectorial electromagnetic fields, associated with orbital angular momentum or nonuniform intensity variations. Here, by considering both spin and angular momentum, we demonstrate a generalized spin-momentum relationship that governs vectorial properties of guided electromagnetic waves, extending optical quantum spin-Hall effect to a two-dimensional vector field of structured guided wave. The effect results in the appearance of the out-of-plane transverse optical spins, which vary progressively from the 'up' state to the 'down' state around the energy flow, and their variation is uniquely locked to the energy propagation direction. The related spin-momentum locking in a chiral spin swirl is demonstrated with four kinds of surface structured waves and proven both theoretically and experimentally. The results provide understanding of the spin dynamics in electromagnetic guided waves and show great importance in spin optics, topological photonics and optical spin-based devices and techniques.