Vertically-Stacked Discrete Plasmonic Meta-Gratings for Broadband Space-Variant Metasurfaces

Reexamining the plasmonic metasurfaces for efficient transmissive nanophotonics devices has recently drawn considerable attention. It attributes to their ease of fabrication and tunability in terms of the excitation of abundant multipoles. Despite recent efforts in developing plasmonic meta-atoms, meta-gratings provide an ultimate solution that enables efficient conversion of the polarization in a broadband range, particularly at optical frequencies. In this work, by vertically stacking meta-gratings whose bandwidths overlap with each other and possess no physical limitation, plasmon mode hybridization is introduced so that highly-transmissive broadband plasmonic metasurfaces are realized. It is reported that the intra-coupling in a discrete plasmonic meta-grating plays a key role in geometric phase-controlled metasurface design. As a proof of concept, a half-wave plate using plasmonic meta-gratings to convert the incident circularly-polarized light in a wavelength range from 1106 to 1521 nm with a peak efficiency of 50.2% and a gradient metasurface with truncated discrete meta-grating as building blocks for deflecting the light with a maximum of 32.60% are demonstrated. Finally, the physical picture is explained through the electric field distributions, mode coupling, and energy splitting. The presented framework may provide an optimized way to enhance efficiency and expand the bandwidth for plasmonic systems.