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.

光子具有自旋自由度，它在各种应用（例如光通信，信息处理和传感）中起着重要作用。在各向同性介质中，光子自旋与光的传播方向对齐，遵循自旋动量锁定原理。有趣的是，远离界面衰减的表面波具有一个垂直于其传播的光子自旋，从而为在受限系统中观察自旋相关的激发提供了令人兴奋的机会。在这里，我们提出并在不依赖任何界面的情况下，在块状介质中实现了横向光子自旋（T-spin）。我们通过将各向异性引入介质中，显示了表面模式的T型自旋到整体模式的T自旋。我们进一步发现，两个方向相反的各向异性介质之间的界面支持具有可调截止频率的边沿传播模式。我们的结果为操纵电磁波的自旋轨道相互作用提供了一个新的平台。