Quantum spin Hall effect of light

TLDR

Maxwell’s equations, formulated 150 years ago, describe light from classical to quantum regimes, giving rise to geometric and topological phenomena linked to photons’ spin‑1 massless nature and leading to evanescent surface modes such as surface plasmon‑polaritons at vacuum‑metal interfaces. The study demonstrates that free‑space light exhibits an intrinsic quantum spin Hall effect, revealing surface modes with strong spin‑momentum locking. The work reveals transverse spin in evanescent waves, explains experiments on spin‑direction locking of surface optical modes, and links Maxwell’s theory to topological insulators, suggesting robust spin‑directional optical interfaces.

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.