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New Phononic Crystal Might Enable Better Mobile Communications

New Phononic Crystal Might Enable Better Mobile Communications
Using a microwave microscope, UT Austin researchers visualized elastic waves as they travel right to left, following a Z-shaped boundary in a phononic crystal, without losing energy. The image is approximately 0.1 millimeters high. Credit: Keji Lai/University of Texas at Austin.

Experts have taken a step towards the use of special materials called phononic crystals in cell phones and other mobile devices, an advance that is important to make more devices compatible with emerging 5G communications and other new technologies.

A team of researchers, including Keji Lai of The University of Texas at Austin, report in the journal Nature Electronics they have designed a novel type of phononic crystal key to the advance.

Cellular devices aren't 100% electronic but rather rely on acoustic components to process information using physical vibrations, or elastic waves. The new acoustic component, made of aluminum nitride and configured into periodic phononic crystals, allows engineers to direct high frequency elastic waves along predefined paths, including sharp turns and splits, without losing signal. It hadn't previously been clear that physical properties would allow for such an advance; this is the first demonstration of a type of physics called "topological protection" in this type of material at gigahertz frequencies.

"This work brings the concept of topology to gigahertz acoustic waves," said Qicheng (Scott) Zhang, a postdoc in the lab of Charlie Johnson at the University of Pennsylvania who led the study. "We demonstrated that we can have this interesting physics at a useful range, and now we can build up the platform for more interesting research to come."

When driven by a device called a transducer, the new phononic crystal quivers like a trampoline with someone jumping on it, except the vibrations are much more subtle. The amplitude or height of the vibrations passing through the membrane are just one tenth the size of an atom. The UT Austin team led by associate professor of physics Keji Lai developed a new way to visualize these miniscule vibrations called microwave impedance microscopy.

"Before this, if people want to see what's going on [in these materials], they usually would need to go to a national lab and use X-rays," Lai said. "It's very tedious, time-consuming and expensive. But in my lab, it's just a tabletop setup and we measure a sample in about 10 minutes, and the sensitivity and resolution are better than before."

Lai's approach uses a commercial atomic force microscope with modifications and additional electronics that he developed.

To begin the project, Bo Zhen, an assistant professor at Penn who has expertise in studying topological properties in light waves, conducted simulations to determine the best types of devices to fabricate. Then, based on the results of the simulations and using high-precision tools at the Singh Center for Nanotechnology, Penn researchers etched nano-scale circuits onto aluminum nitride membranes, essentially making novel phononic crystals.

"When you're conveying a wave along a sharp trail, in ordinary systems that don't have these topological effects, at every sharp turn you're going to lose something, like power, but in this system you don't," said Charlie Johnson, the Rebecca W. Bushnell Professor of Physics and Astronomy at Penn.

Overall, the researchers say that this work provides a critical starting point for progress in both fundamental physics research as well as for developing new devices and technologies. In the near term, the researchers are interested in modifying their device to make it more user-friendly and improving its performance at higher frequencies, including frequencies that are used for applications such as quantum information processing.

"In terms of technological implications, this is something that could make its way into the toolbox for 5G or beyond," adds Johnson. "The basic technology we're working on is already in your phone, so the question with topological vibrations is whether we can come up with a way to do something useful at these higher frequency ranges that are characteristic of 5G."

Other collaborators on the project are based at Beijing University of Posts and Telecommunications.

This research was supported by the National Science Foundation, the Welch Foundation and the U.S. Office of Naval Research.

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Wednesday, 25 May 2022

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