  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.
