Picking up on acoustic signals through better filtration

In the context of sensory modalities, eyes work like tiny antennae, picking up light, electromagnetic waves traveling at blistering speeds. When humans look at the world, their eyes catch these waves and convert them into signals the brain reads as colors, shapes, and movement. It’s a seamless process, that allows people to see details clearly even when there's a lot happening around them.

Ears, on the other hand, act more like microphones, capturing sound through vibrations in the air. When someone speaks, sound waves hit the eardrums, vibrating and sending signals to the brain. But unlike the clarity eyes offer, ears can struggle in noisy environments, where many different types of sounds may be overlapping.

Yue Jiang, a Ph.D. student in the Charlie Johnson Group at the University of Pennsylvania, compares this challenge to what scientists face when trying to filter sound in modern technology. “We need ways to isolate important signals from the noise, especially with wireless communication becoming so essential,” Jiang says. “With countless signals coming from many directions, it’s easy for interference to interfere with the transmission.”

To that end, Jiang and her team in the Johnson Group have developed a way to control sound waves using a process called Klein tunneling, applied in a high-frequency range.

“What’s exciting about this is that we’ve pushed Klein tunneling—the movement of particles like electrons through an energy barrier—to the gigahertz range,” says Charlie Johnson. “These are the frequencies your cell phone operates at, so our findings could lead to faster, more reliable communication systems.”

The team's findings, published in the journal Cell Devices, mark the first time Klein tunneling has been demonstrated with sound waves at such high frequencies, paving the way for more efficient, faster, noise-resistant communication systems, and it has implications for quantum information systems, where precise control of sound is critical. By fine-tuning how sound waves travel, the research could lead to more reliable wireless communication and advanced technologies.

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At the core of their research are phononic crystals, engineered materials designed to manipulate sound waves in a way similar to how photonic crystals control light. The team etched “snowflake-like” patterns onto ultra-thin membranes made from aluminum nitride, a piezoelectric material that converts electrical signals into mechanical waves and vice versa, and these patterns play a crucial role in guiding sound waves through Dirac points, which allow them to pass through energy barriers with minimal energy loss.

The membranes, only 800 nanometers thick, were designed and fabricated at Penn’s Singh Center for Nanotechnology. “The snowflake patterns let us fine-tune how waves travel through the material,” Jiang says, “helping us reduce unwanted reflections and increase signal clarity.”

To confirm their results, the researchers collaborated with Keji Lai’s Research Group at the University of Texas at Austin using transmission-mode microwave impedance microscopy (TMIM) to visualize sound waves in real time. “TMIM allowed us to see these waves moving through the crystals at gigahertz frequencies, giving us the precision needed to confirm Klein tunneling was happening,” Jiang says.

The team’s success builds on previous work with Lai’s lab, which explored controlling sound waves at lower frequencies. “Our earlier work with Keji helped us understand wave manipulation,” Johnson says. “The challenge was extending that understanding to much higher frequencies.”

In recent experiments, the team demonstrated near-perfect transmission of sound waves at frequencies between 0.98 GHz and 1.06 GHz. By controlling the angle at which the waves entered the phononic crystals, they could guide the waves through barriers with little energy loss, making their method a highly effective way to filter and direct sound signals.

As the team members move forward, they are exploring the potential applications of their findings in areas like 6G wireless communication, where the demand for faster data transmission and less interference is critical. “By controlling sound waves more precisely, we could allow more users to connect simultaneously in densely populated frequency bands,” Jiang says.

They are also testing new materials, such as scandium-doped aluminum nitride, which could enhance the effect of Klein tunneling and offer even better performance at higher frequencies. “We’re pushing the limits to see how far we can extend these principles,” Jiang says, “and how they can be applied to both classical and quantum technologies.”

Ultimately, the researchers hope to develop ultra-precise, angle-dependent filters for a variety of applications, including wireless communication, medical imaging, and quantum computing. “This research is just the beginning,” Johnson says. “We’re setting the stage for a new generation of acoustic devices that could really change how we think about sound wave transmission and control.”

Charlie Johnson is the Rebecca W. Bushnell Professor of Physics and Astronomy in the Department of Physics and Astronomy in Penn’s School of Arts & Sciences.

Yue Jiang is a Ph.D. student in the Charlie Johnson Group in Penn Arts & Sciences.

Keji Lai is a professor in the Department of Physics at the University of Texas at Austin.

Other researchers involved in the study are Chengyu Wen of the University of Pennsylvania’s School of Engineering and Applied Science; Shahin Jahanbani, Daehun Lee, and Xiaoru Zhang of the University of Texas at Austin; and Qicheng Zhang of Westlake University.

The research was supported by the National Science Foundation (Grant NSF DMR-1720139), U.S. Department of Defense (Grant FA9550-20-1-0105), and Research Center for Industries of the Future at Westlake University. Additional support came from the Singh Center for Nanotechnology at the University of Pennsylvania, which is part of the National Nanotechnology Coordinated Infrastructure (Grant NNCI-2025608).