Abstract
Metasurfaces have manifested unprecedented capabilities in manipulating light by subwavelength unit cells, facilitating the miniaturization and multifunctions of optical systems. On the other hand, lithium niobate on insulator (LNOI) technology is revolutionizing the integrated photonics, enabling multifunctional devices and applications. Yet the optical interface for coupling and manipulation is not sufficient and versatile. Here, we developed a geometric metasurface interface for LNOI waveguide and demonstrated several on-chip integrated devices for free space light field manipulations. By decorating waveguides with subwavelength optical antennas, we manipulated the guided waves into desired wavefronts in space, achieved complex free-space functions, such as focusing, multichannel vortex beam generation, and holography. Our architecture goes beyond the conventional gratings and enriches the functionalities of metasurface, which would open up a new perspective for future versatile guided-wave driven optical devices.
Highlights
The development of integrated photonics empowers people to integrate light sources, modulators, optical antennas, and other functional components in a tiny chip for in-plane light routing and out-of-plane radiation
Subwavelength antennas with different rotation angle are patterned on the top surface of the lithium niobate on insulator (LNOI) waveguide, and the guided waves are coupled out with circular polarization and manipulated into desired free-space light according to the geometric phase design
As a proof of concept, we design an integrated lens consists of 60 Si nanorods with f = 15 μm at 1550 nm, the center-to-center distance between adjacent nanorod is 400 nm, numerical aperture (NA) is 0.625
Summary
The development of integrated photonics empowers people to integrate light sources, modulators, optical antennas, and other functional components in a tiny chip for in-plane light routing and out-of-plane radiation. Ni et al demonstrated a hybrid metasurface directly driven by guided wave to realize different free space functions based on the silicon waveguide [32]. We developed an integrated geometric metasurface in LNOI platform as the interface between the guided wave and free-space radiation light with versatile functions. Subwavelength antennas with different rotation angle are patterned on the top surface of the LNOI waveguide, and the guided waves are coupled out with circular polarization and manipulated into desired free-space light according to the geometric phase design. We experimentally fabricated geometric silicon (Si) metasurfaces on the LN slab waveguide by electron beam lithography and reactive ion etching process, and demonstrated functions of integrated metalens focusing, multichannel vortex beam generation, and holographic imaging. Our approach breaks the limit of traditional gratings for guided wave couplers, and extends the guided wave based geometric metasurface, which show apparent advantages compared with the commonly used resonant metasurfaces
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