We show that multi-level diffractive microstructures can enable broadband, on-axis transmissive holograms that can project complex full-color images, which are invariant to viewing angle. Compared to alternatives like metaholograms, diffractive holograms utilize much larger minimum features (>10 µm), much smaller aspect ratios (<0.2) and thereby, can be fabricated in a single lithography step over relatively large areas (>30 mm ×30 mm). We designed, fabricated and characterized holograms that encode various full-color images. Our devices demonstrate absolute transmission efficiencies of >86% across the visible spectrum from 405 nm to 633 nm (peak value of about 92%), and excellent color fidelity. Furthermore, these devices do not exhibit polarization dependence. Finally, we emphasize that our devices exhibit negligible absorption and are phase-only holograms with high diffraction efficiency.

We utilized nonlinear optimization to design phase-only diffractive lenses that focus light to a spot, whose width is smaller than that dictated by the far-field diffraction limit. Although scalar-diffraction theory was utilized for the design, careful comparisons against rigorous finite-difference time-domain simulations confirm the superfocusing effect. We were able to design a lens with a focal spot size that is 25% smaller than that formed by a conventional lens of the same numerical aperture. An optimization strategy that allows one to design such lenses is clearly explained. Furthermore, we performed careful simulations to elucidate the effects of fabrication errors and defocus on the performance of such optimized lenses. Since these lenses are thin, binary, and planar, large uniform arrays could be readily fabricated enabling important applications in microscopy and lithography.

A comprehensive simulation model of the performance of photochromic films in absorbance-modulationoptical-lithography AIP Advances 6, 035210 (2016); 10.1063/1.4944489Outdoor measurements of a photovoltaic system using diffractive spectrum-splitting and concentration AIP Advances 6, 095311 (2016) Absorbance-Modulation-Optical Lithography (AMOL) has been previously demonstrated to be able to confine light to deep sub-wavelength dimensions and thereby, enable patterning of features beyond the diffraction limit. In AMOL, a thin photochromic layer that converts between two states via light exposure is placed on top of the photoresist layer. The long wavelength photons render the photochromic layer opaque, while the short-wavelength photons render it transparent. By simultaneously illuminating a ring-shaped spot at the long wavelength and a round spot at the short wavelength, the photochromic layer transmits only a highly confined beam at the short wavelength, which then exposes the underlying photoresist. Many photochromic molecules suffer from a giant mismatch in quantum yields for the opposing reactions such that the reaction initiated by the absorption of the short-wavelength photon is orders of magnitude more efficient than that initiated by the absorption of the long-wavelength photon. As a result, large intensities in the ring-shaped spot are required for deep sub-wavelength nanopatterning. In this article, we overcome this problem by using the long-wavelength photons to expose the photoresist, and the short-wavelength photons to confine the "exposing" beam. Thereby, we demonstrate the patterning of features as thin as λ/4.7 (137nm for λ = 647nm) using extremely low intensities (4-30 W/m 2 , which is 34 times lower than that required in conventional AMOL). We further apply a rigorous model to explain our experiments and discuss the scope of the reverse-AMOL process. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

A proximity-effect-correction (PEC) algorithm for three-dimensional (3D) single-photon gray-scale photolithography is proposed and numerically analyzed in this paper. The gray-scale dose assigned to every point within the photoresist volume is optimized to guarantee that the fabricated 3D patterns are as close to the designed patterns as possible. PEC optimizations for 3D woodpile geometries using low and high absorption photoresist are simulated. Spatial resolution of the proposed PEC algorithm is numerically studied. We also investigated the efficacy of our algorithm on a variety of related 3D geometries.

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