Broadband point-spread function engineering via a freeform diffractive microlens array

We utilized inverse design to engineer the point-spread function (PSF) of a low-f-number, freeform diffractive microlens in an array, so as to enable extended depth of focus (DOF). Each square microlens of side 69 mu m and focal length 40 mu m (in a polymer film, n similar to 1.47) generated a square PSF of side similar to 10 mu m that was achromatic over the visible band (450 to 750 nm), and also exhibited an extended DOF of similar to +/- 2 mu m. The microlens has a geometric f/# (focal length divided by aperture size) of 0.58 in the polymer material (0.39 in air). Since each microlens is a square, the microlens array (MLA) can achieve 100% fill factor. By placing this microlens array (MLA) directly on a high-resolution print, we demonstrated integral imaging with applications in physical security. The extended DOF preserves the optical effects even with expected film-thickness variations, thereby increasing robustness in practical applications. Since these multi-level diffractive MLAs are fabricated using UV-nanoimprint lithography, they have the potential for low-cost large volume manufacturing. (C) 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement