  The ability to control the directionality of surface waves by manipulating
its polarization has been of great significance for applications in spintronics
and polarization-based optics. Surface waves with evanescent tails are found to
possess an inherent in-plane transverse spin which is dependent on the
propagation direction while an out-of-plane transverse spin does not naturally
occur for surface waves and requires a specific surface design. Here, we
introduce a new type of surface waves called chiral surface wave which has two
transverse spins, an in-plane one that 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. The two transverse spins are locked to the
momentum. 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.
