A mm-sized optical lens made from a hybrid glass material demonstrated the ability to change its refractive behavior in the presence of gas. Developed by researchers at the University of Jena, the lens’ behavior stems from the materials from which it is made. The molecular structure of the lens consists of a 3D lattice with cavities that can accommodate gas molecules, which affects the optical properties of the material. The team derived the hybrid glass from zeolitic imidazolate framework (ZIF-62), a type of metal organic framework (MOF). According to Lothar Wondraczek, a professor of glass chemistry at the University of Jena, the MOFs his team used are currently being researched and developed as materials to store or separate gas. One of the challenges facing Wondraczek’s team, which is developing multi-responsive materials, is how to adapt classical glass-forming techniques to hybrid MOF materials. “Most of these substances decompose when heated and are therefore very difficult to form,” researcher Oksana Smirnova said. Additionally, the quality of the MOF materials starting out, and the resulting processed glasses, can affect optical purity, spatial homogeneity, and achievable specimen size. Researchers from the University of Jena led by Lothar Wondraczek are investigating optical microlenses made from hybrid glasses. Courtesy of Jens Meyer/University of Jena. To create an optical microlens made with hybrid glasses, the team first had to develop a suitable synthesis process to ensure highly pure materials, and then had to identify the optimal conditions under which the material could be shaped. The team created a procedure for the liquid handling of high-quality ZIF-62 glass and optimized the synthetic conditions necessary to achieve high-quality ZIF-62(Zn) crystals. They reevaluated the melting processes for agZIF-62(Zn), i.e., glassy ZIF-62(Zn). The researchers evaluated the optical properties of agZIF-62(Zn) using homogeneous, highly transparent cm-scale samples that were free of any residual contamination, bubbles, cracks, or grain boundaries that could compromise optical analyses. To provide a proof of concept for the optical application of this material, they used thermal micro-imprinting to generate concave and convex micro-optical components. “We melt the material and then transfer it into a 3D-printed mold, where it is pressed,” Alexander Knebel, junior research group leader at the Chair of Glass Chemistry, said. “This process allows us to choose almost any desired shape.” Smirnova said that the team specifically chose the lens as the shape for the 3D-printed, hybrid glass-based optical component because even the smallest impurities are noticeable in a lens, as they directly affect the lens’s optical properties. Using 3D-printed fused silica templates, the researchers showed that concave, as well as convex, lens structures could be obtained at high precision by remelting the glass, without trade-offs in material quality. The process to imprint lenses from ZIF-62 hybrid glasses could be used to make micro-optical devices that combine the gas uptake and permeation capabilities of MOFs with the optical functionality of glass. As an example, the researchers demonstrated the reversible change of optical refraction on the incorporation of volatile guest molecules. The change in the refractive behavior of the lens in the presence of gas indicated that the hybrid-glass lens refracted light more or less strongly, depending on whether gas was absorbed in the lens material. Optical-quality glasses derived from ZIF-62 in the form of cm-scale objects could be used for in-depth studies of optical transparency and refraction across the ultraviolet to near-infrared spectral range. The process of shaping the bulk agZIF-62(Zn) into the lens structures provided the team with a perspective on the potential uses of MOF-glasses in photonic devices. The new process for imprinting micro-optical components from optical-quality hybrid glasses allows for a wide variety of shapes and geometries, extending beyond the specific application as micro-lenses. “Because these multi-responsive materials react to multiple influences simultaneously, they could be used for logical circuits, for example,” Wondraczek said. “This specifically means that two conditions are linked for the observable reaction. If a light beam hits the lens and gas is absorbed in the lens material simultaneously, then the light is refracted in a particular way, providing combined feedback.” Wondraczek said that membranes for gas separation, whose optical properties change when gas molecules are present, are also a potential application for optical components printed from hybrid glass. Such components could be used in smart, multi-responsive sensor technology, making measurement methods more efficient, space-saving, and intelligent. The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-49428-1).
