Optical Gap-Surface Plasmon Metasurfaces for Spin-Controlled Surface Plasmon Excitation and Anomalous Beam Steering

Multifunctional metasurfaces featuring diversified functionalities offer unprecedented possibilities for developing versatile ultracompact micro/nanophotonic devices and systems. Until recently, most multifunctional metasurfaces were designed for light incidence with orthogonal linear polarizations, being unable to function with orthogonal circularly polarized (CP) light, which is of vital importance in spin photonics, chiroptical spectroscopy/imaging, and optical sensing. Here we consider the design of efficient spin-decoupled multifunctional gap-surface plasmon (GSP) gradient metasurfaces and experimentally demonstrate simultaneous spin-controlled unidirectional surface plasmon polariton (SPP) excitation and anomalous beam steering in the optical regime under orthogonal right and left CP light incidence, respectively. The spin-decoupled GSP gradient metasurface, consisting of rotated GSP-based nanoscale half-wave plates, combines both propagation and geometric phases to produce two different spin-dependent linear phase gradients enabling SPP excitation and anomalous reflection. The proof-of-concept fabricated metasurface exhibits broadband (850-950 nm) operation featuring efficient (>22%) unidirectional SPP excitation and high-efficiency (48% on average) anomalous beam steering for right and left CP incident light, respectively. Our demonstration of metasurface-enabled spin-controlled unidirectional SPP excitation along with free-propagating beam steering opens new possibilities for spin photonics and plasmonics with potential applications ranging from biomedical diagnosis, chiroptical spectroscopy, and imaging to optical sensing.