  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.
