Merging Geometric Phase and Plasmon Retardation Phase in Continuously Shaped Metasurfaces for Arbitrary Orbital Angular Momentum Generation

Traditional technologies to generate and manipulate the orbital angular momentum (OAM) suffer from bulky size and do not lend themselves to nanophotonic systems. An ultrathin metasurface based on abrupt phase shift has recently been proposed as an alternative method. Nevertheless, gradient phase was generally approximated by multiple meta-atom/molecules with discrete levels of abrupt phase shift, which not only increases the design and fabrication complexity but also causes difficulties in obtaining simultaneous electrical and optical functionality. Furthermore, a discontinuous phase profile would introduce phase noise to the scattering fields and deteriorate the purity of the OAM beams. Here, we propose a wavefront engineering mechanism by virtue of the spin-orbit interaction in continuously shaped plasmonic metasurfaces, which offers a new approach to generate OAM modes of high purity. Equally important, a method producing arbitrary OAM topological charge, integral and fractional, is demonstrated by merging the plasmon retardation phase with the geometric phase in the proposed continuously shaped metasurfaces. The proposed approach is well supported by full-wave numerical simulations and experimental characterization of the fabricated structure in the visible regime.