Controlling sound waves with Klein tunneling improves acoustic signal filtration

In the context of sensory modalities, the eyes function like tiny antennas, capturing light and electromagnetic waves traveling at lightning speeds. When humans look at the world, their eyes pick up these waves and convert them into signals that the brain interprets as colors, shapes, and movement. It is a transparent process that allows users to see details clearly, even when there is a lot going on 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, vibrate and send signals to the brain. But unlike the clarity that the eyes provide, the ears can struggle in noisy environments, where many different types of sounds can overlap.

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

To this end, Jiang and his team at the Johnson Group developed a way to control sound waves using a process called Klein tunneling, applied in a high frequency range.

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

The team’s work, published in the journal Devicemarks the first time that Klein tunneling has been demonstrated with sound waves at such high frequencies, paving the way for more efficient, faster, and more noise-resistant communications systems, and has implications for communication systems. quantum information, where precise control of sound is essential. By refining the way sound waves propagate, the research could lead to more reliable wireless communications and advanced technologies.

At the heart of their research are phononic crystals, materials designed to manipulate sound waves in the same way that photonic crystals control light. The team etched “snowflake-like” patterns onto ultra-thin membranes made of 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 enable them to cross energy barriers with minimal energy loss.

The membranes, just 800 nanometers thick, were designed and fabricated at Penn’s Singh Center for Nanotechnology.

“Snowflake patterns allow us to fine-tune how waves propagate through the material,” says Jiang, “thus helping us reduce unwanted reflections and increase signal clarity.”

To confirm their findings, 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 frequencies on the order of gigahertz, which gave us the precision needed to confirm that Klein tunneling was occurring,” says Jiang.

The team’s success builds on previous work with Lai’s lab, which explored the control of sound waves at lower frequencies. “Our previous work with Keji helped us understand wave manipulation,” says Johnson. “The challenge was to extend this 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 the barriers with little energy loss, making their method a very effective way of filtering and directing sound signals.

As team members move forward, they are exploring potential applications of their findings in areas such as 6G wireless communication, where the demand for faster data transmission and less interference is essential.

“By controlling sound waves more precisely, we could enable more users to connect simultaneously in densely populated frequency bands,” says Jiang.

They are also testing new materials, such as scandium-doped aluminum nitride, which could enhance Klein tunneling and provide even better performance at higher frequencies. “We’re pushing the boundaries to see how far we can extend these principles,” says Jiang, “and how they can be applied to 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 are laying the groundwork for a new generation of acoustic devices that could truly change the way we think about the transmission and control of sound waves.”