Title: FDTD-based Inverse Design enables f/0.27 flat Microlens Array for Integral Imaging

Abstract: We demonstrate a high-NA (0.88), ultra-low-f-number (f/0.2714), multi-wavelength (480nm, 550nm and 650nm) multilevel diffractive MicroLens Array (MLA) using inverse design. Each microlens in the array is close-packed with diameter of 70 {\mu}m and focal length of only 19 {\mu}m in air. The MLA was patterned on one surface of a polymer film via UV casting, such that the focal plane was located on the distal end of the film (n of polymer ~ 1.47, thickness = 28 {\mu}m, effective f/# (NA) inside polymer ~ 0.4 (0.78)). Each microlens focuses incident light at 3 design wavelengths into a focal spot with measured full-width at half-maximum (FWHM) < 1 {\mu}m. By placing this MLA directly on a high-resolution print, we demonstrated RGB integral imaging with applications in document security. Compared to refractive MLAs, our diffractive MLA reduces the thickness by > 3X, which is advantageous for manufacturability. Since these multi-level diffractive MLAs are fabricated using UV-casting, they have the potential for low-cost, high-volume manufacturing.