Hello,

We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not an Insider? Subscribe now for unlimited access to online articles.

Rewriting Life

First Acoustic Superlens

An ultrasound lens could be used for high-resolution clinical imaging.

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.

In focus: When filled with water, the holes in this aluminum plate act as resonant cavities that can focus ultrasound waves.

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.

The latest Insider Conversation is live! Listen to the story behind the story.

Subscribe today
Already a Premium subscriber? Log in.

Uh oh–you've read all of your free articles for this month.

Insider Premium
$179.95/yr US PRICE

More from Rewriting Life

Reprogramming our bodies to make us healthier.

Want more award-winning journalism? Subscribe to Insider Plus.
  • Insider Plus {! insider.prices.plus !}*

    {! insider.display.menuOptionsLabel !}

    Everything included in Insider Basic, plus the digital magazine, extensive archive, ad-free web experience, and discounts to partner offerings and MIT Technology Review events.

    See details+

    What's Included

    Unlimited 24/7 access to MIT Technology Review’s website

    The Download: our daily newsletter of what's important in technology and innovation

    Bimonthly print magazine (6 issues per year)

    Bimonthly digital/PDF edition

    Access to the magazine PDF archive—thousands of articles going back to 1899 at your fingertips

    Special interest publications

    Discount to MIT Technology Review events

    Special discounts to select partner offerings

    Ad-free web experience

/
You've read all of your free articles this month. This is your last free article this month. You've read of free articles this month. or  for unlimited online access.