Penn State Researchers Create New Material to Focus Acoustic Waves

In a cover article in The Journal of Applied Physics, a team of Penn State
researchers has designed and computationally tested a type of manmade
metamaterial capable for the first time of manipulating a variety of acoustic
waves with one simple device. This invention will benefit almost all current
sonic and ultrasonic applications, such as ultrasonic nondestructive evaluations
and ultrasonic imaging. The device should also provide more accurate and
efficient high-intensity focused ultrasound(HIFU) therapies, a non-invasive
heat-based technique targeted at a variety of cancers and neurological
disorders.

Optical metamaterials have been widely studied in the past decade for
applications such as cloaking and perfect lenses. The basic principles of optical
metamaterials apply to acoustic metamaterials. Artificial structures are created
in patterns that bend the acoustic wave onto a single point, and then refocus
the acoustic wave into a wider or narrower beam, depending on the direction of
travel through the proposed acoustic beam aperture modifier. The acoustic
beam aperture modifier is built upon gradient-index phononic crystals, in this
case an array of steel pins embedded in epoxy in a particular pattern. The
obstacles (steel pins) slow down the acoustic wave speed in order to bend the
acoustic waves into curved rays.

According to post-doctoral scholar and the paper’s lead author, Sz-Chin Steven
Lin, while other types of acoustic metamaterials also could focus and defocus
an acoustic beam to achieve beam aperture modification (although prior to this
work no such beam modifier has been proposed), their device possesses the
advantage of small size and high energy conservation. Currently, researchers
and surgeons need to have many transducers of different sizes to produce
acoustic waves with different apertures. This is analogous to having to swap out
lenses on a camera to change the lens’s aperture. With this invention, by
changing the modifier attached to the transducer the desired aperture can be
easily attained.

"Design of acoustic beam aperture modifier using gradient-index phononic
crystals," by Lin, Bernhard Tittmann, and Tony Jun Huang, is the first design
concept for an acoustic beam aperture modifier to appear in the scientific
literature, and no acoustic beam modifier device is available in the market. As a
result, the authors expect their device could have wide applications across
several important acoustic fields, from medical ultrasound to higher sensitivity
surface acoustic wave sensors to higher Q factor resonators. The team is
currently making a prototype based on this design.

Support for their research came from the National Science Foundation, the
National Institutes of Health (NIH) Director’s New Innovator Award, and the Penn
State Center for Nanoscale Science (MRSEC). Sz-Chin Steven Lin is a post-
doctoral scholar in the Penn State Department of Engineering Science and
Mechanics. Bernhard Tittmann is Schell professor and professor of engineering
science and mechanics. Tony Jun Huang is associate professor of engineering
science and mechanics.

The Materials Research Institute is Penn State’s home for interdisciplinary
materials research, supporting over 220 engineers and scientists and 800-plus
graduate students, post-docs and visiting scientists. Visit MRI at
www.mri.psu.edu for other recent materials discoveries at Penn State.