A fairly new technique, two-photon polymerization, was developed to print high-resolution micro-optics based on organic polymers. But organic optics printed by polymer-based components tend to be somewhat limited in practical applications because of their poor thermal and chemical stability, as well as low transmission in short and long wavelengths, and low refractive indices.
Glass optical elements are necessary to meet these needs for ultraviolet (UV), visible, near-infrared (NIR), and infrared (IR) wavelengths. And micro-optical elements are too small (<1 mm) to be fabricated via traditional optical fabrication methods, so extra steps such as fabricating molds for optical molding are needed to fabricate smaller optics.
This inspired Professor Rongguang Liang’s lab in the Wyant College of Optical Sciences and Professor Douglas Loy’s lab in the Department of Chemistry and Biochemistry at the University of Arizona to develop a photosensitive liquid silica resin (LSR) based on a precondensed silica commonly used to prepare low-density silica aerogels. And when PhD students Zhihan Hong and Piaoran Ye combined it with the two-photon polymerization printing method, they ended up with complex optical elements with high accuracy (see figure).
“3D printing is the perfect solution for smaller and complex optics. Transparent glass optics can be created via thermal treatment at 600°C, with shrinkage as low as 17%,” says Liang. “This is the lowest reported shrinkage rate for printing optics, but the goal is to go even smaller. We’ll optimize the material and printing process to further reduce the shrinkage rate.”
Shrinkage is caused by the burnout of organics and melting of particles into a glass during the sintering process. “For high-performance imaging applications, optical elements should be fabricated to specification,” he adds. “Smaller shrinkage is better for more accurately controlling the final shape. The challenge now is to optimize the LSR for faster curing speeds, better mechanical properties, lower thermal treatment temperatures, and a lower shrinkage rate.”
Compared to traditional polishing and molding methods, this method can be used to fabricate glass optical elements in freeform and discontinuous shapes, as well as complex multi-element alignment-free optical systems with movable elements.
“Our material and printing method has a significant impact on translating advanced imaging techniques from the lab into clinical endoscopic imaging by enabling rapid fabrication of high-performance glass micro-optics not possible before,” says Liang. “Precision glass micro-optics will also enable new imaging techniques in UV, visible, and IR regions for various applications.”
How long does it take to print complex optical elements? For a micro-objective with a 0.5 mm diameter, it takes about 2 hours to print and another 20 minutes to remove the uncured material with propylene glycol methyl ether acetate (PGMEA).
The most immediate application is for biomedical imaging—from miniaturized endoscopes to wearable sensors. Thanks to its unique properties in thermal stability, mechanical properties, chemical resistance, and imaging performance within UV, visible, NIR, and IR regions, other potential applications range from consumer imaging to integrated photonics and space optics.
“In the future, we’ll continue to develop new materials with higher refractive indices, faster curing speed, and a lower shrinkage rate,” says Liang. “Our work opens the door for novel optics systems and applications not possible before, such as micro-optical systems with moving elements. It will also catalyze academic research in materials development.”