Chiral Surface Wave Propagation with Anomalous Spin-Momentum Locking

Photon spin has received great interest in recent decades for many applications such as encoding quantum information and spin-filtering. However, very little is known about controlling the direction and properties of the spin. It was recently found that surface waves with evanescent tails possess an inherent in-plane transverse spin that is dependent on the propagation direction, while an out-of-plane transverse spin does not naturally occur and requires a specific surface design. Here, we introduce a new type of surface wave called a chiral surface wave, which has two transverse spins, an in-plane one, which is inherent to any surface wave, and an out-of-plane spin, which is enforced by the design due to strong x-to-y coupling and broken rotational symmetry. We show that the two transverse spins are locked to the momentum, providing a highly confined spin-dependent propagation. Our study opens a new direction for metasurface designs with enhanced and controlled spin-orbit interaction by adding an extra degree of freedom to control the propagation direction as well as the transverse spin of surface waves.