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.