High-efficiency focused optical vortex generation with geometric gap-surface plasmon metalenses

Optical vortices (OVs) carrying orbital angular momentum have been an emerging research area due to their widespread applications ranging from optical communication to particle trapping. However, conventional OV generators suffer from bulky configurations and limited performance. Here, we design and experimentally demonstrate high-efficiency focused OV generation by using geometric gap-surface plasmon metalenses with spin-flipped focal planes, where the polarization states, positions, and corresponding topological charges of the OVs can be controlled by changing the spin of incident circularly polarized light, reproducing thereby the combined functionalities of a lens and a q-plate. The fabricated dual-polarity OV metalens features the polarization conversion ratios above 88% and efficiencies exceeding 50% for both right-handed and left-handed circularly polarized light in a wide wavelength range of 850 to 1000nm. We further extend this approach to realize a multiple-focal OV metalens with distinct and spin-multiplexed OVs at four different longitudinal focal planes. Compared to conventional OV generators, the proposed OV metalenses are compact, ultrathin, and multiplexed, therefore opening an innovative avenue for advanced research and applications aiming at multiple functionalities and dense integration in photonics.