Boosting the frequency of sound waves to make the next generation of wireless devices

Vincent Kerler, a second-year physics major in the College of Arts and Sciences, says he spent his summer running "a whole bunch of simulations to explore how mechanical waves move through a class of materials that convert mechanical stress into electricity and vice versa, or piezoelectric nanomaterials."

His goal? To identify the best crystal structures and material compositions that could reduce reflections and lost power, thereby enhancing the performance of mechanical waves in wireless communications devices like cell phones.

Under the guidance of Yue Jiang, a Ph.D. candidate in the Charlie Johnson research groupin the School of Arts & Sciences, Kerler conducted this work through the Penn Undergraduate Researching Mentoring Program, a 10-week opportunity from the Center for Undergraduate Research and Fellowships. The program provides rising second- and third-year students with $5,000 awards to work alongside Penn faculty.

"What we want is to have the wave propagate through the crystal without really bouncing back at that boundary because that's where energy gets lost," says Kerler, who is from Columbus, Ohio. "So, I used software to model the behavior of the waves in our materials and specifically looked at how crystal lattice structures, sizes, and gradients impact their properties and reduce reflection at the boundaries."

Specifically, Kerler is looking at ultrathin films, where their extent is much larger than their actual thickness, that have a coupling between the mechanical stress and their electromagnetic properties.

"If you deform them, you get a voltage across them, or, if you put a voltage across them, there might be deformation," Kerler says.

"This new, exciting area of condensed matter physics," Johnson says, "pretty much started with two of my Penn colleagues, Gene Meleand Charlie Kane, who popularized the concept of topological insulators. These are materials where the topology-a kind of geometry that focuses on the properties of space that are preserved under continuous transformations-plays a critical role in how electrons move within a crystal."

Their work laid the foundation for a new understanding of quantum mechanics, "but it didn't stop there," Johnson says, "as researchers soon realized that the same principles could be applied to other types of waves, not just electrons."

Johnson explains that his lab is interested in getting these mechanical waves, which are essentially sound waves the researchers call phonons, to operate at higher frequencies. The higher frequencies are analogous to the electromagnetic spectrum, the set of wave-based data transmission that radios, mobile phones, and other wireless devices use.

"What Vincent learns from these simulations might possibly be useful in current and future generations of cell phones because the frequency of the sound wave that we're working with right now is of the order of one gigahertz, which is right in a cell phone band," Johnson says.

Though the work involved hours of running simulations, often waiting as long as three hours for results, Kerler found it rewarding.

"It's a slow process, but seeing patterns form in the data is really exciting," he says. "My enthusiasm for the project really grew as I saw how small tweaks in the crystal structure could lead to significant changes in wave behavior. Again, it's not fast work, and there were moments where I had to scrap an entire week's worth of tests, but when things start to come together it's so worth it."

Looking ahead, the next steps in Kerler's research involve further refining these simulations to achieve even more precise control over wave behavior. As Johnson points out, "If we could do better at bringing the energy in, then that would make all of our experiments better." This could involve transitioning from the theoretical models that Kerler is working with to physical experiments at the Singh Center for Nanotechnology's nanofabrication facility.

In addition, the team is exploring the use of advanced materials, such as scandium-doped aluminum nitride, which offer the promise of even higher piezoelectric coupling and, therefore, higher frequency resonances.

"There's still so much more to explore, and I'm really fascinated by the potential of these materials," Kerler says.