Over the past few years, researchers have developed several materials that bend light in ways that appear to violate the laws of physics, creating so-called superlenses, for ultra-high-resolution optical imaging, as well as invisibility cloaks. Now researchers have demonstrated that the same kind of images and cloaking devices could be made with sound instead of light. Using the first acoustic metamaterial ever produced, the researchers were able to focus ultrasound waves. This represents a significant step toward creating high-resolution ultrasound images and cloaking devices capable of hiding ships from sonar.
Acoustic lenses can be made to focus sound much as the lens in a microscope focuses light. But physicists’ ability to work with both types of waves is limited by scattering effects called diffraction. Using conventional lenses, it’s not possible to focus light waves or sound waves to a spot size smaller than half the wavelength of the light. To get around these limitations, a lens must refract, or literally bend light backward. No naturally occurring materials have a negative index of refraction, but some materials carefully designed in the lab, called metamaterials, do. The same tools used to make materials that can focus light or sound waves beyond the diffraction limit, enabling high-resolution imaging, can also be used to make materials that accomplish the opposite, cloaking an object by directing light or sound around it.
Theorists have been working on materials that bend sound waves backward for several years. Such a metamaterial has now been built by Nicholas Fang, an assistant professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. His group’s sound-focusing device is an aluminum array of narrow-necked resonant cavities whose dimensions are tuned to interact with ultrasound waves. The cavities are filled with water. Fang likens them to an array of wind instruments, such as the pipes in an organ. When ultrasound waves move through the array, the cavities resonate so that the sound is focused. The cavities “work together to refract the sound,” says Fang.
“This is a big step forward for acoustic metamaterials,” says Steven Cummer, an associate professor of electrical and computer engineering at Duke University. Cummer was involved with the development of the first optical cloaking device. “It’s a good experimental confirmation that ideas from electromagnetics can be extended to acoustics,” he says. “Figuring out a good way to do this experimentally was not easy.”
The ultrasound system, described in the journal Physical Review Letters, hasn’t yet exceeded the diffraction limit. But researchers expect Fang to beat it soon. “I am sure that we shall not have long to wait,” says John Pendry, a professor of theoretical solid-state physics at Imperial College London, who designed the materials used by Duke researchers to make the first invisibility cloak.
“There are many important applications awaiting a successful sub-wavelength acoustical focusing device,” says Pendry. The first application of acoustic metamaterials is likely to be in high-resolution clinical ultrasound imaging, says Fang. “Without pumping more energy into tissue, you can provide a sharper image.” However, he notes that applications are a ways off. “We’ve done focusing, but not yet imaging,” says Fang.