Liquid Silica Resin Optimizes 3D Printing for Complex Micro-Optics

A liquid silica resin (LSR) developed by researchers at the University of Arizona has proven successful as a 3D-printing medium for complex micro-optics. The organic-inorganic hybrid material has shown high curing speeds, better mechanical properties, lower thermal treatment temperatures, and reduced shrinkage. Inorganic silica glass can be achieved by thermally treating the printed sample at 600 °C in air.

For the 3D-printing of optics and micro-optics, the silica resin is an alternative to organic polymers. Despite their lightweight and low-cost qualities, organic polymer-based optics do not offer the same level of thermal stability, chemical resistance, and imaging performance in the UV and IR bands as that which can be delivered by optics based on inorganic glass.

Researchers at the University of Arizona have used an LSR as a 3D-printing medium for complex micro-optics. This SEM image, of a printed glass Alvarez lens, shows an optic with an element (on the right) that can be moved to change the power of the lens. Courtesy of Hong et al.

Traditional grinding and polishing methods used in optical fabrication are far from the most effective when it comes to micro-optics and are not an option for freeform micro-optics. Precision glass molding has been the method of choice, though it's not without limitations. Multielement components and freeform optics with microstructures pose a barrier.

Additive manufacturing has made progress in that respect, though the resulting optics often suffer from lower resolution and require further refinement in post-processing. Two-photon polymerization (TPP) methods have shown particular promise, as the resolution obtained has been much better. Previous work from the Arizona researchers on TPP-based additive manufacturing with LSR has demonstrated success, but certain problems remain.

In the current work, the team refined its existing LSR formula used in 3D-printing applications, after noticing a tendency for it develop deformations in specific applications. This deformation occurred most often during the printing and thermal treatment processes when creating a structure with high aspect ratio, mainly due to the relatively low number of crosslinked points in the printed structure.

When printing a lens objective with a 50-μm diameter and a height of 100 μm, the supporting structure lacked the strength to support the whole objective once the printing process was completed.

To remedy this, the team focused on creating a series of LSRs with increased crosslinkable points, by adjusting the ratio of methacryloxymethyltrimethoxsilane (MMTS) during synthesis. The team varied the concentration of MMTS between 6.5 and 20 mol%. The researchers found that increasing the concentration of MMTS also increased shrinkage during thermal treatment.

To achieve the high quality needed for optics manufacturing, the shrinkage must be low to better control the shape and surface quality. The researchers therefore needed to find a balance between an increased level of MMTS for better crosslinking and the resulting increase in shrinkage. While the sample with 20 mol% of MMTS had better mechanical properties, the team ultimately chose the sample with 15 mol% as it had less shrinkage.

The printing method developed alongside the material is capable of printing almost all types of optical surfaces, the researchers said, including flat, spherical, aspherical, freeform, and discontinuous surfaces, with accurate surface shape and high surface quality for imaging applications.

The researchers demonstrated this by printing a number of structures that would not have been possible with their previous formula for LSR. This included lenses with Fresnel structures, a micro-objective with three elements, an Alvarez lens, and other elements. Lenses created with this method were able to clearly resolve images, including a focused image of a housefly’s wing.

Based on the measured surface quality and shape deviation of the optics the team created, as well as the image quality they obtained, the researchers believe that 3D printing of glass imaging optics will play a significant role in precision optical imaging. Potential applications span the ability to fabricate optics with greater flexibility for endoscopes and microspectrometers, as well as the development of freeform micro-optics, complex multielement alignment-free optical systems, and optical systems with moving elements.

The research was published in Advanced Science (www.doi.org/10.1002/advs.202105595).