Ultra-thin, planar, Babinet-inverted plasmonic metalenses

We experimentally demonstrate the focusing of visible light with ultra-thin, planar metasurfaces made of concentrically perforated, 30-nm-thick gold films. The perforated nano-voids-Babinet-inverted (complementary) nano-antennas-create discrete phase shifts and form a desired wavefront of cross-polarized, scattered light. The signal-to-noise ratio in our complementary nano-antenna design is at least one order of magnitude higher than in previous metallic nano-antenna designs. We first study our proof-of-concept 'metalens' with extremely strong focusing ability: focusing at a distance of only 2.5 mu m is achieved experimentally with a 4-mu m-diameter lens for light at a wavelength of 676 nm. We then extend our work with one of these 'metalenses' and achieve a wavelength-controllable focal length. Optical characterization of the lens confirms that switching the incident wavelength from 676 to 476 nm changes the focal length from 7 to 10 mm, which opens up new opportunities for tuning and spatially separating light at different wavelengths within small, micrometer-scale areas. All the proposed designs can be embedded on-chip or at the end of an optical fiber. The designs also all work for two orthogonal, linear polarizations of incident light.