  The study of spin-orbit coupling (SOC) of light is crucial to explore the
light-matter interactions in sub-wavelength nanostructures with broken
symmetries. In noncentrosymmetric photonic crystals, the SOC results in the
splitting of the otherwise degenerate energy bands. Herein, we explore the SOC
in a noncentrosymmetric plasmonic crystal, both theoretically and
experimentally. Cathodoluminescence (CL) spectroscopy combined with the
numerically calculated photonic band structure reveals an energy band splitting
that is ascribed to the broken symmetries in the noncentrosymmetric plasmonic
crystal. By shifting the impact position of the electron beam throughout a unit
cell of the plasmonic crystal, we show that the emergence of the energy band
splitting strongly depends on the excitation position of the surface plasmon
(SP) waves on the crystal. Moreover, we exploit angle-resolved CL and
dark-field polarimetry to demonstrate polarization-dependent scattering of SP
waves interacting with the plasmonic crystal. The scattering direction of a
given polarization is determined by the transverse spin angular momentum
inherently carried by the SP wave, which is in turn locked to the direction of
SP propagation. Our study gives insight into the design of novel plasmonic
devices with polarization-dependent directionality of the Bloch plasmons. We
expect spin-orbit plasmonics will find much more scientific interests and
potential applications with the continuous development of nanofabrication
methodologies and uncovering new aspects of spin-orbit interactions.
