Additive fabrication of SiO2-based micro-optics with lag-free depth and reduced roughness.

Ultra-thin optical components with high design flexibility are required for various applications in today's optical and imaging systems, and this is why the use of diffractive optical elements (DOEs) is rapidly increasing. They can be used for multiple optical systems because of their compact size, increased design flexibility, and ease of mass production. Unfortunately, most existing DOEs are fabricated using conventional etching-based methods, resulting in high surface roughness and aspect ratio-dependent etching rate. Furthermore, when small feature size and large feature size patterns co-exist in the same DOE design, the etching depth differs significantly in the same design, called reactive-ion etching (RIE) lag. All these artifacts lead to a reduction in the diffraction efficiency of DOEs. To overcome the drawbacks of etching-based fabrication methods, we propose an alternative method for fabricating DOEs without RIE lag and with improved surface smoothness. The method consists of additively growing multilevel microstructures of SiO2 material deposited by the plasma-enhanced chemical vapor deposition (PECVD) method onto the substrate followed by liftoff. We demonstrate the effectiveness of the fabrication methods with representative DOEs for imaging and laser beam shaping applications.

Hexagonal diffractive optical elements.

Diffractive optical elements (DOEs) have widespread applications in optics, ranging from point spread function engineering to holographic display. Conventionally, DOE design relies on Cartesian simulation grids, resulting in square features in the final design. Unfortunately, Cartesian grids provide an anisotropic sampling of the plane, and the resulting square features can be challenging to fabricate with high fidelity using methods such as photolithography. To address these limitations, we explore the use of hexagonal grids as a new grid structure for DOE design and fabrication. In this study, we demonstrate wave propagation simulation using an efficient hexagonal coordinate system and compare simulation accuracy with the standard Cartesian sampling scheme. Additionally, we have implemented algorithms for the inverse DOE design. The resulting hexagonal DOEs, encoded with wavefront information for holograms, are fabricated and experimentally compared to their Cartesian counterparts. Our findings indicate that employing hexagonal grids enhances holographic imaging quality. The exploration of new grid structures holds significant potential for advancing optical technology across various domains, including imaging, microscopy, photography, lighting, and virtual reality.

Etch-free additive lithographic fabrication methods for reflective and transmissive micro-optics.

With the widespread application of micro-optics in a large range of areas, versatile high quality fabrication methods for diffractive optical elements (DOEs) have always been desired by both the research community and by industry. Traditionally, multi-level DOEs are fabricated by a repetitive combination of photolithography and reactive-ion etching (RIE). The optical phase accuracy and micro-surface quality are severely affected by various etching artifacts, e.g., RIE lag, aspect ratio dependent etching rates, and etching artifacts in the RIE steps. Here we propose an alternative way to fabricate DOEs by additively growing multi-level microstructures onto the substrate. Depth accuracy, surface roughness, uniformity and smoothness are easily controlled to high accuracy by a combination of deposition and lift-off, rather than etching. Uniform depths can be realized for both micrometer and millimeter scale features that are simultaneously present in the designs. The grown media can either be used directly as a reflective DOE, or as a master stamp for nanoimprinting refractive designs. We demonstrate the effectiveness of the fabrication methods with representative reflective and transmissive DOEs for imaging and display applications.

Computational imaging using lightweight diffractive-refractive optics.

Diffractive optical elements (DOE) show great promise for imaging optics that are thinner and more lightweight than conventional refractive lenses while preserving their light efficiency. Unfortunately, severe spectral dispersion currently limits the use of DOEs in consumer-level lens design. In this article, we jointly design lightweight diffractive-refractive optics and post-processing algorithms to enable imaging under white light illumination. Using the Fresnel lens as a general platform, we show three phase-plate designs, including a super-thin stacked plate design, a diffractive-refractive-hybrid lens, and a phase coded-aperture lens. Combined with cross-channel deconvolution algorithm, both spherical and chromatic aberrations are corrected. Experimental results indicate that using our computational imaging approach, diffractive-refractive optics is an alternative candidate to build light efficient and thin optics for white light imaging.