Transverse photon spin of bulk electromagnetic waves in bianisotropic media

Photons possess spin degree of freedom, which plays an important role in various applications such as optical communications, information processing and sensing. In isotropic media, photon spin is aligned with the propagation direction of light, obeying the principle of spin momentum locking. Interestingly, surface waves decaying away from an interface have a photon spin transverse to its propagation, opening exciting opportunities for the observation of spin-dependent excitation in confined systems. Here, we propose and realize transverse photon spin (T-spin) in a bulk medium, without relying on the presence of any interfaces. We show the mapping of the T-spin of surface modes to that of the bulk modes by introducing bianisotropy into the medium. We further discover that the interface between 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.