Metasurface optics for full-color computational imaging

Abstract

Conventional imaging systems comprise large and expensive optical components that successively mitigate aberrations. Metasurface optics offers a route to miniaturize imaging systems by replacing bulky components with flat and compact implementations. The diffractive nature of these devices, however, induces severe chromatic aberrations, and current multiwavelength and narrowband achromatic metasurfaces cannot support full visible spectrum imaging (400 to 700 nm). We combine principles of both computational imaging and metasurface optics to build a system with a single metalens of numerical aperture ~0.45, which generates in-focus images under white light illumination. Our metalens exhibits a spectrally invariant point spread function that enables computational reconstruction of captured images with a single digital filter. This work connects computational imaging and metasurface optics and demonstrates the capabilities of combining these disciplines by simultaneously reducing aberrations and downsizing imaging systems using simpler optics.

Fig. 1. Design, simulation, and fabrication of imaging metasurfaces.

(A) The metasurfaces are made up of silicon nitride nanoposts, where the thickness T, lattice constant p, and diameter d are the design parameters. (B) Schematic of a metasurface comprising an array of nanoposts. (C) Simulation of the nanoposts’ transmission amplitude and phase via RCWA. Simulated intensity along the optical axis of the singlet metasurface lens (D) and EDOF metasurface (E), where, going from top to bottom in each panel, 400, 550, and 700 nm wavelengths are used. The dashed lines indicate the desired focal plane where the sensor will be placed. Optical images of the singlet metasurface lens (F) and the EDOF device (G). Scale bars, 25 μm.

Fig. 2. Characterization of the imaging metasurfaces.

The PSFs of the singlet metalens (top row) and EDOF lens (bottom row) were measured under blue (A and E), green (B and F), and red (C and G) illumination conditions. Scale bars, 25 μm. The MTFs were also calculated for both designs (D and H). In both (D) and (H), a normalized frequency of 1 corresponds to the same cutoff frequency of 579 cycles/mm.

Fig. 3. Imaging at discrete wavelengths.

The appropriately cropped original object patterns used for imaging are shown in (A) and (B). Images were captured of the 1951 Air Force resolution chart with the singlet metalens (C) and the EDOF lens without (D) and with deconvolution (E). Images were also taken of a binary Mona Lisa pattern with the singlet metalens (F) and the EDOF device without (G) and with deconvolution (H). Scale bars, 20 μm.

Fig. 4. Imaging with white light.

Images were taken under white light illumination of color printed RGB (A) and ROYGBIV (B) text, a colored rainbow pattern (C), and picture of a landscape (D) with a blue sky, green leaves, and multicolor flowers. The appropriately cropped original object patterns used for imaging are shown in the left column. Scale bars, 20 μm.