Manufacturing Bits: Dec. 9

Metalens breakthroughs
Using a conventional lithography system, Harvard has developed what researchers call an all-glass, centimeter-scale metalens.

A metalens is a flat surface, which makes use of nanostructures to focus light. It’s a disruptive technology that could displace traditional glass-based lenses. Applications include virtual reality (VR) devices, biological imaging techniques and others.

A traditional lens is made from glass. Conventional lenses, such as refractive lenses, are bulky and require sophisticated manufacturing processes, according to a recent paper from Michigan State University.

Potentially, a metalens could replace bulky, curved lenses currently used in optical devices. “A metasurface-based flat lens (metalens) holds promise in wave-front engineering for multiple applications,” according to Michigan State University.

The problem? A metalens is unable to focus the full spectrum of light. Plus, a metalens is typically the size of a piece of glitter, making it impractical for most applications.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have paved the wave toward the development of practical metalens structures.

Using traditional deep-ultraviolet (DUV) projection lithography, Harvard demonstrated a way to make 45 metalenses, which are 1cm diameter on a 4-inch fused-silicon wafer.

This in turn enables an all-glass, centimeter-scale metalens in the visible spectrum. “The lenses show diffraction-limited focusing behavior for any homogeneously polarized incidence at visible wavelengths. The metalens’ performance is quantified by the Strehl ratio and the modulation transfer function (MTF), which are then compared with commercial refractive spherical and aspherical singlet lenses of similar size and focal length,” according to Nano Letters, a technical journal.

“Previously, we were not able to achieve mass-production of centimeter-scale metalenses at visible wavelengths because we were either using electron-beam lithography, which is too time consuming, or a technique called i-line stepper lithography, which does not have enough resolution to pattern the required subwavelength-sized structures,” said Joon-Suh Park, a Ph.D. candidate at SEAS.

“This research paves the way for so-called wafer level cameras for cell phones, where the CMOS chip and the metalenses can be directly stacked on top of each other with easy optical alignment because they are both flat,” said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “In the future, the same company can make both the chip and the lenses because both can be made using the same technology: lithography.”

3D glass printing
Using a 3D printing process, ETH Zurich has developed a way to make complex and porous glass objects.

So far, researchers have developed smaller 3D-printed glass objects, which are the size of dice. Ultimately, the goal is to one day develop bottles, drinking glasses or window panes.

Several groups are attempting to make glass using 3D printing. Some are using molten glass, but this requires high-temperature processes. Another way is to use materials at room temperature, but this way produces only simple objects.

Researchers from ETH Zurich have developed a method based on stereolithography. Researchers have also developed a special resin, which consists of plastic and organic molecules.

The resin is processed using UV light patterns. The light hits the resin and it hardens. “The plastic monomers combine to form a labyrinth-like structure, creating the polymer. The ceramic-bearing molecules fill the interstices of this labyrinth,” according to ETH Zurich.

An object can be built layer by layer. “Here, we report a digital light-processing 3D printing platform that exploits the photopolymerization-induced phase separation of hybrid resins to create glass parts with complex shapes, high spatial resolutions and multi-oxide chemical compositions,” according to ETH in the journal Natural Materials. “Analogously to conventional porous glass fabrication methods, we exploit phase separation phenomena to fabricate complex glass parts displaying light-controlled multiscale porosity and dense multicomponent transparent glasses with arbitrary geometry using a desktop printer.”