Researchers have discovered topological phenomena occurring at frequency relevant to modern technology

_{Microwave microscope images a topological acoustic wave that has been perfectly transmitted, as shown in the schematic. According to a recent study from the laboratory of Charlie Johnson and colleagues, topological control possibilities in an acoustic system at high technologically relevant frequencies are described, with implications for 5G communications and quantum information processing. The image is courtesy: Qicheng Zhang}

Authors in Nature Electronics disclose topological control capabilities in an integrated acoustic-electronic system operating at frequencies relevant to the real-world application. It opens the door to further study of the qualities of high-frequency sound waves’ topological features for use in 5G communications and quantum information processing, among other things. Researchers from Charlie Johnson’s lab, the Beijing University of Posts and Communications, and the University of Texas in Austin collaborated to conduct the work, which was conducted by postdoc Qicheng (Scott) Zhang.

Penn researchers Charlie Kane and Eugene Mele devised a theoretical framework known as topological materials, which this discovery relies on. On the inside, topological insulators operate as an electrical insulator, yet their surfaces are electrically conducting. Materials that utilise light or sound waves, rather than electricity, have been proposed to exhibit topological phenomena.

Topological phononic crystals, metamaterials that utilise acoustic waves, or phonons, were of particular interest to Zhang in this work. Because of the relevance of gigahertz frequencies in future telecommunications technologies like 5G, Zhang sought to determine if topological events might also arise at higher frequencies in the megahertz range in these crystals.

As part of their investigation into this complicated system, the researchers used cutting-edge techniques in theory, modelling and nanofabrication. Before fabricating any devices, researchers at the Zhen lab, who are experts in researching light waves’ topological characteristics, ran simulations to figure out which devices would be most effective. The researchers next etched nanoscale circuits onto aluminium nitride membranes using high-precision technologies from Penn’s Singh Center for Nanotechnology. UT Austin’s Keji Lai’s group used microwave impedance microscopy, a technique that captures high-resolution pictures of ultra-small acoustic waves, on these devices. In Lai’s technique, a commercial atomic force microscope is modified and supplemented with electronic components invented in his lab in order to get the desired results.

It had previously been necessary to go to a national laboratory and utilise X-rays to examine what was going on in these materials. “It takes a lot of time, effort, and money. Tabletop setups in my lab take about 10 minutes to perform the same measurements as they did before; the sensitivity is improved as well.”

Topological events may be observed at higher frequencies, as demonstrated experimentally in this study. In Zhang’s words, “this study introduces the notion of topology to acoustic waves.” Now that we’ve shown that we’re capable of having this fascinating physics within a practical range, we can lay the groundwork for even more fascinating research in the future.”

The atomic structure of the device can be used to incorporate these properties, allowing different parts of the material to transmit signals in different ways. These results were predicted by theorists, but Johnson calls them “amazing” when he first saw them in action. When a wave travels along a sharp path in an ordinary system without these topological effects, it loses power at every sharp turn; in this system, it doesn’t. “That also has its own important implications,” he says.

It’s generally agreed upon by the researchers that this work provides a critical starting point for progress in fundamental physics research as well as the creation of new devices and technologies. This device will be improved to be more user-friendly in the near future, with a focus on making it perform better at higher frequencies, such as those required for quantum information processing.

“This is something that could make its way into the toolbox for 5G or beyond,” says Johnson in terms of technological implications. It’s a question of whether we can do something useful at the higher frequency ranges characteristic of 5G with the topological vibrations technology we’re working on, because the basic technology is already in your phone.

Further information: Qicheng Zhang et al, Gigahertz topological valley Hall effect in nanoelectromechanical phononic crystals, Nature Electronics (2022). DOI: 10.1038/s41928-022-00732-y

Journal information: Nature Electronics

Source: University of Pennsylvania