Quantum spin Hall effect of light

TL;DR

This paper reveals that free-space light exhibits an intrinsic quantum spin Hall effect with surface modes showing strong spin-momentum locking, deepening understanding of Maxwell's theory and suggesting new optical applications.

Contribution

It demonstrates the quantum spin Hall effect in light, linking Maxwell's equations to topological phenomena and explaining recent experimental observations.

Findings

Surface modes with strong spin-momentum locking identified.

Transverse spin in evanescent waves explained.

Analogies with topological insulators established.

Abstract

Maxwell's equations, formulated 150 years ago, ultimately describe properties of light, from classical electromagnetism to quantum and relativistic aspects. The latter ones result in remarkable geometric and topological phenomena related to the spin-1 massless nature of photons. By analyzing fundamental spin properties of Maxwell waves, we show that free-space light exhibits an intrinsic quantum spin Hall effect, i.e., surface modes with strong spin-momentum locking. These modes are evanescent waves that form, e.g., surface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate the unusual transverse spin in evanescent waves and explain recent experiments demonstrating the transverse spin-direction locking in the excitation of surface optical modes. This deepens our understanding of Maxwell's theory, reveals analogies with topological insulators for electrons, and offers applications for robust spin-directional optical interfaces.