Metamaterial focuses sound waves like a camera lens

Ashley N. Rice, ashley.rice@photonics.com

A computationally tested metamaterial that can manipulate a variety of acoustic waves with one simple device holds promise for various acoustic applications, including medical ultrasound, higher sensitivity surface acoustic wave sensors and higher Q factor resonators.

Man-made optical metamaterials have been studied over the past decade for applications including perfect lenses and cloaking. 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 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 can effectively shrink or expand the aperture of an acoustic beam with minimum energy loss and waveform distortion. With such an acoustic lens, the need for a series of expensive transducers of various sizes is eliminated. Courtesy of Sz-Chin “Steven” Lin, Penn State.

“The acoustic beam aperture modifier is a brand-new application of acoustic metamaterials that has not been built before,” Sz-Chin “Steven” Lin, a postdoctoral scholar at The Pennsylvania State University, told BioPhotonics.

Lin and colleagues at the university’s Materials Research Institute built the novel device on gradient-index (GRIN)phononic crystals – in this case, an array of steel pins embedded in epoxy in a particular pattern. The steel pins, or obstacles, slow the acoustic wave speed so that they can be bent into curved rays.

“The acoustic beam aperture modifier is built upon gradient-index phononic crystals which are artificially engineered periodic structures most famous for their ability to guide the propagation of acoustic waves along curved trajectories, known as the acoustic mirage effect,” said Lin, lead author of the report, which appeared in the Journal of Applied Physics (http://dx.doi.org/10.1063/1.4729803).

Although other types of acoustic metamaterials could focus and defocus an acoustic beam to achieve aperture modification, the Penn State device is smaller in size by at least half and offers energy conservation of up to 83 percent of acoustic energy after modification.

“Compared to existing negative-refraction-based metamaterial lenses, our GRIN-based metamaterial lens possesses several advantages,” Lin said. “First, the GRIN lens can operate over a wide frequency band, while negative-refraction-based lenses usually operate within a small range.

“Second, a GRIN lens can be coupled with acoustic transducers and can effectively redirect paraxial incident acoustic waves to a small focal spot – the position of this spot is determined by the adjustable gradient coefficient. In contrast, with negative-refraction-based phononic crystal lenses, one must focus select diverging waves to a long focal zone.”

Lastly, GRIN lenses can be made much smaller than negative-refraction-based lenses and can be seamlessly integrated with existing millimeter-scale acoustic systems, he said.

For the past several years, Lin has worked to apply optics concepts such as GRIN lensing to the phononic crystals. He has applied his GRIN concept to various fields, including optofluidics and nanophotonics, to obtain optical lenses.

Currently, scientists and surgeons must have transducers of multiple sizes to produce acoustic waves with different apertures. With the new device, the desired aperture can be attained easily by changing the modifier attached to the transducer.

The device will benefit almost all sonic and ultrasonic applications, including evaluations and imaging. It also could provide more accurate and efficient high-intensity focused ultrasound therapies, a noninvasive heat-based technique targeted at a variety of cancers and neurological disorders.

The team is working on a prototype based on this design.