Physicists at The University of Texas at Dallas and Wuhan University have created an artificial structure that does not reflect sound and bends it in a way that does not occur in nature.
The results could inspire new directions in wave manipulation, such as acoustic cloaking technologies, and advances in photonics and electronics, said Dr. Fan Zhang, assistant professor of physics at UT Dallas and one of the authors of the study published in Nature.
Zhang, a theoretical physicist, and his collaborators first conceived of a novel structure called a Weyl sonic crystal. Such a crystal has unique properties that theoretically permit acoustic waves to exhibit zero reflection and negative refraction as they hit the interface between two facets of the crystal.
“Because of the negative refraction and zero reflection of sound at the interface of two facets, this type of crystal could act kind of like a cloaking device for sound,” Zhang said.
Refraction occurs when waves such as light or sound bend slightly as they travel from one medium to another. To envision this, think about light from a flashlight shining down from the air onto the surface of water below it. Part of the light beam reflects off the surface, and part passes into the water. The part of the beam that goes into the water bends, or refracts, into a slightly different path from the original beam.
Optical devices like cameras and microscopes use lenses that rely on refraction to direct and refocus light into clear images. Many acoustic devices work the same way, but with sound.
During negative refraction, however, a beam of light or sound is bent back in a mirror-image direction from what is normally expected. Although negative refraction does not occur in nature, researchers have engineered artificial substances with negative refraction. Such metamaterials are particularly useful for cloaking and superlensing applications.
“There is still some reflection associated with these metamaterials. But we have found a fundamentally new route to make it zero,” Zhang said.
“We have demonstrated that it is possible to construct a sonic crystal to manipulate sound in a way that is forbidden in nature. This new knowledge could open up some important applications not only in acoustics but also in photonics and electronics.”
Dr. Fan Zhang, assistant professor of physics in the School of Natural Sciences and Mathematics
During a trip to his hometown, Zhang teamed with experimentalists at Wuhan University to construct the novel Weyl sonic crystal, roughly a cube about the size of a desktop computer.
“To test our theory, we created an artificial crystal that is structurally simple and can be fabricated by a 3D printer,” Zhang said.
First, the team 3D-printed individual rectangular rods of a plastic material, then stacked them in layers, like building blocks, in a “woodpile” configuration. Each layer, consisting of several rods arranged parallel with one another, was offset by 120 degrees from the layer beneath it.
After squaring off the sides, the researchers launched a sound signal at the surface of their unconventional Weyl sonic crystal. Microphones inserted inside the structure mapped how the sound wave was dispersed as it traveled along different facets of the crystal.
As the sound wave hit an edge separating two adjacent facets, no sound was reflected back along the incident facet, while the wave exhibited negative refraction along the refractive facet.
“We have demonstrated that it is possible to construct a sonic crystal to manipulate sound in a way that is forbidden in nature,” Zhang said. “This new knowledge could open up some important applications not only in acoustics but also in photonics and electronics.”
The research was published in the August 1, 2018, issue of Nature. Efforts at Wuhan University were supported by the National Science Foundation of China. Zhang’s efforts were supported by UT Dallas research enhancement funds from the School of Natural Sciences and Mathematics.
Zhang recently received a grant from the U.S. Army Research Office to explore exotic physics in 2D materials and to develop innovative concepts in electronics.