Metasurface Creates Ultrathin Tiny Planar Lenses
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 µm is achieved experimentally with a 4-μm-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 μm, 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.
Schematic designs and the results of full-wave simulations of the individual Babinet-inverted nano-antennas at a wavelength of 676 nm. The nano-antennas create discrete phase shifts from 0 to 7π/4 for cross-polarized light. The linearly-polarized light enters the system from the glass substrate side of the sample. The pseudo-color field map indicates the cross-polarized light scattered from each nano-antenna, clearly revealing the discrete phase shifts.
Comparison between the measured and simulated results for three different metalens designs at a wavelength of 676 nm. (a) Results for a metalens designed for a focal length of 2.5 µm. (b) Results for sample B designed for a focal length of 5 µm. (c) Results for sample C designed for a focal length of 7 µm. (a1, a2, b1, b2, c1, and c2) Reconstructed cross-polarized light intensity distribution on the transmission side of the metalenses as derived from measurements; (a3, a4, b3, b4, c3, and c4) Simulated results for the same designs. (a1, a3, b1, b3, c1, and c3) Intensity distributions for two cross-sectional planes cutting through the center of the metalens. (a2, a4, b2, b4, c2, and c4) Intensity distribution at the respective focal planes (z coordinates are shown on the plots). The x–y planes in (a1), (b1), and (c1) are at z=0 µm. The x–y planes in (a3), (b3), and (c3) are at z=0.1 µm (avoiding the singularity at z=0 µm in the simulations). The effect of the depth of focus of the objective lens has been taken into account in the simulations by averaging the intensity data in the z-direction within a 0.5-μm window.
Reference: X. Ni, S. Ishii, A. V. Kildishev, and V. M. Shalaev, “Ultra-thin, planar, Babinet-inverted plasmonic metalenses,” Light: Science & Applications, vol. 2, e72, 2013. [Fulltext | Link]