Physicists map new route to control sound in thin films

Postdoctoral researcher Pragalv Karki and Jayson Paulose, an assistant professor of physics, focused on mechanical vibrations in thin elastic plates, the building blocks for their proposed design of synthetic films known as metamaterials, using theoretical and computational analysis. They also developed a simpler model consisting of springs and masses to demonstrate the signal manipulation ability.

"There have been a lot of mechanisms that can guide or block the transmission of sound waves through a metamaterial, but our design is the first to dynamically stop and reverse a sound pulse," Karki said.

The interplay between bending stiffness and global tension, two physical parameters governing sound transmission in thin plates, is at the heart of their mechanism. While bending stiffness is a material property, global tension is an externally controllable parameter in their system.

Karki and Paulose of the UO Department of Physics and Institute for Fundamental Science described their mechanism, which they call dynamic dispersion tuning, in a paper published online March 29 in the journal Physical Review Applied.

"If you throw a stone onto a pond, you see the ripples," Karki said. "But what if you threw the stone and instead of seeing ripples propagating outward you just see the displacement of the water going up and down at the point of impact? That's similar to what happens in our system."

The ability to manipulate sound, light or any other wave in artificially made metamaterials is an active area of research, Karki said.

Optical metamaterials, which exhibit properties such as a negative refractive index not possible with conventional materials, were initially developed to control light in ways that could be used to create invisibility cloaks and superlenses. Their use is being explored in diverse applications such as aerospace and defense, consumer electronics, medical devices and energy harvesting.

Acoustic metamaterials are usually static and unchangeable, so how to tune their properties is an ongoing challenge, Karki said. Other research groups have proposed several strategies, such as using origami-inspired designs or magnetic switching.

"In our case, the tunability comes from the ability to change the tension of the drumlike membranes in real time," Karki said.

Additional inspiration, Karki and Paulose noted, came from research in the UO lab of physicist Benjamín Alemán that was published in Nature Communications in 2019. Alemán's group unveiled a graphene nanomechanical bolometer, a drumlike membrane that can detect colors of light at high speeds and high temperatures. The approach exploits a change in global tension.

While the mechanism in the new paper was identified theoretically and needs to be proven in lab experiments, Karki said, he is confident the approach will work.

"Our mechanism of dynamic dispersion tuning is independent of whether you are using acoustic, light or electronic waves," Karki said. "This opens up the possibility of manipulating signals in photonic and electronic systems as well."

The approach, he said, could include improved acoustic signal processing and computation. Designing acoustic metamaterials based on graphene, such as those in Alemán's lab, could be useful in such technologies as wave-based computing, micromechanical transistors and logic devices, waveguides, and ultrasensitive sensors.