Transverse photon spin beyond interfaces

TL;DR

This paper introduces and experimentally demonstrates the concept of transverse photon spin within bulk media, revealing new ways to control spin-orbit interactions of light without relying on interfaces.

Contribution

It presents the first realization of transverse photon spin in bulk media through bianisotropic coupling, expanding the understanding of photon spin beyond surface-bound phenomena.

Findings

Transverse photon spin can be achieved inside bulk media.

Bianisotropic coupling maps surface mode spin to bulk modes.

Interfaces between opposite bianisotropic media support tunable edge modes.

Abstract

Photons possess spin degree of freedom, corresponding to clockwise and counter clockwise rotating direction of the fields. Photon spin plays an important role in various applications such as optical communications, information processing and sensing. In conventional isotropic media, photon spin is aligned with the propagation direction of light, obeying spin momentum locking. Interestingly, at certain interfaces, the surface waves decaying away from the interface possess a photon spin transverse to its propagation, opening exciting opportunities for observation of spin dependent unidirectional excitation in confined systems. Here we propose and realize transverse photon spin (T-spin) in the interior of a bulk medium, without relying on the presence of any interfaces. We show the complete mapping of the T-spin of surface modes to that of the bulk modes by introducing the coupling between electric and magnetic responses along orthogonal directions, i.e., the bianisotropy, into the medium. We further discover that an interface formed by two bianisotropic media of opposite orientations supports edge-dependent propagating modes with tunable cutoff frequencies. Our results provide a new platform for manipulating the spin orbit interaction of electromagnetic waves.