Gap-surface plasmon metasurfaces for linear-polarization conversion, focusing,and beam splitting

Gap-surface plasmon(GSP)metasurfaces have attracted progressively increasing attention due to their planar configurations,ease of fabrication,and unprecedented capabilities in manipulating the reflected fields that enable integrating diverse bulk-optic-based optical components into a single ultrathin flat element.In this work,we design and experimentally demonstrate multifunctional metalenses that perform simultaneous linear-polarization conversion,focusing,and beam splitting,thereby reproducing the combined functionalities of conventional halfwave plates,parabolic reflectors,and beam splitters.The fabricated single-focal metalens incorporates properly configured distinct half-wave-plate-like GSP meta-atoms and exhibits good performance under linearly polarized incidence in terms of orthogonal linear-polarization conversion(>75%)and focusing(overall efficiency>22%)in the wavelength spectrum ranging from 800 to 950 nm.To further extend the combined functionalities,we demonstrate a dual-focal metalens that splits and focuses a linearly polarized incident beam into two focal spots while maintaining the capability of orthogonal linear-polarization conversion.Furthermore,the power distribution between two split beams can readily be controlled by judiciously positioning the incident beam.The demonstrated multifunctional GSP-based metalenses mimic the combined functionalities of a sequence of discrete bulk optical components,thereby eliminating the need for their mutual alignment and opening new perspectives in the development of ultracompact and integrated photonic devices.