Researchers from the Singapore University of Technology and Design (SUTD) and Southern University of Science and Technology (SUSTech) in China have proposed a fabrication technique that integrates oscillation-assisted digital light processing (DLP) 3D printing with grayscale UV exposure to render an ultrafast and flexible fabrication of microlens arrays with optical surface smoothness. Optically smooth microlens array is fabricated by an oscillation-assisted 3D-printing method. Owing to the mechanical oscillation applied on the projection lens, jagged surface induced by the gaps between discrete pixels is successfully eliminated to render a low surface roughness of 1 nm, which endows the printed microlenses with high resolution. Courtesy of SUTD & SUSTech. With increasing demand for miniaturization of optoelectronics, microlens arrays have attracted significant attention and are widely used in compact imaging, sensing, optical communication, and other applications. Typically, microlens array consists of multiple micron-size lenses with optical surface smoothness and superior uniformity, which increases the requirement for machining precision. Despite the tremendous progress made in manufacturing techniques during the past decades, some limitations, such as high time consumption, high process complexity, lack of fabrication flexibility, and difficulty in consistency control for the existing techniques, still exist. “3D printing of small geometries with optical surface smoothness is a big challenge,” said project leader Qi Ge, an associate professor at SUSTech. In traditional 3D printing, which builds in layers, materials are subjected to what is known as “the staircase effect” where layer marks become distinctly visible on the surface. “In our approach, the computationally designed grayscale patterns are employed to realize microlens profiles upon one single UV exposure, which removes the staircase effect existing in the traditional layer-by-layer 3D-printing method,” Ge said. Through oscillation, the projection lens can remove jagged surfaces that have formed due to the gaps between pixels. The researchers performed detailed morphology characterizations including scanning electron microscopy (SEM) and atomic force microscopy (AFM) to prove that the integration of projection lens oscillation considerably smooths the lens surface and reduces the surface roughness from 200 nm to about 1 nm. “In addition to surface roughness, lens profile also plays a key role in optical performance,” researcher Chao Yuan said. To better assist the grayscale design for microlens array fabrication, the research team developed a theoretical model to describe the photopolymerization process and predict the lens profile. Flexibility is a critical advance in the fabrication of microlens arrays through the implementation of DLP-based 3D printing. “Microlenses with different sizes, geometries, and profiles are printable upon one single UV exposure with different grayscale patterns,” researcher Kavin Kowsari said. In comparison to traditional methods of DLP-based printing fabrication method, Ge claims his team’s oscillation-assisted DLP-based printing method is energy- and time-efficient without degradation of optical performance, which is convenient for commercialization and deployment into mass production. According to Ge, the new approach provides instructive inspirations for other manufacturing fields with high demands for ultrasmooth surfaces. The research was published in ACS Applied Materials & Interfaces (https://doi.org/10.1021/acsami.9b14692).