Berkeley National Laboratory (Berkeley Lab) researchers have devloped an acoustic hyperlens which provides an eightfold boost in the magnification power of sound-based imaging technologies. Clever physical manipulation of the imaging sound waves enables the hyperlens to resolve details smaller than one sixth the length of the waves themselves, bringing into view much smaller objects and features than can be detected using today’s technologies.
“We have successfully carried out an experimental demonstration of an acoustic hyperlens that magnifies sub-wavelength objects by gradually converting evanescent waves into propagating waves,” said Xiang Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of the Nano-scale Science and Engineering Center at the University of California, Berkeley. “Our acoustic hyperlens relies on straightforward cutoff-free propagation and achieves deep subwavelength resolution with low loss over a broad frequency bandwidth.”
Zhang is the corresponding author on a paper reporting this research in the journal Nature Materials. The paper is entitled, “Experimental Demonstration of an Acoustic Magnifying Hyperlens.” Co-authoring this paper with Zhang were Jensen Li, Lee Fok, Xiaobo Yin and Guy Bartal.
Zhang and his co-authors fashioned their acoustic hyperlens from 36 brass fins arranged in the shape of a hand-held fan. Each fin is approximately 20 centimeters long and three millimeters thick. The fins, embedded in the brass plate from which they were milled, extend out from an inner radius of 2.7 centimeters to an outer radius of 21.8 centimeters, and span 180 degrees in the angular direction.
“As a result of the large ratio between the inner and outer radii, our acoustic hyperlens compresses a significant portion of evanescent waves into the band of propagating waves so that the image obtained is magnified by a factor of eight,” says co-author Fok, a graduate student in Zhang’s lab. “We chose brass as the material for the fins because it has a density about 7,000 times that of air, a large ratio that is needed to achieve the strong anisotropy required for a flat dispersion of the sound waves.”
The current version of their acoustic hyperlens successfully produced 2-D images of objects down to 6.7 times smaller than the wavelength of the imaging sound wave. Now Zhang and his team are up-grading their technique to produced 3-D images. They are also working to make their acoustic hyperlens compatible with pulse-echo technology, which is the basis of both medical ultrasounds and underwater sonar imaging systems.
“Directly applied to current ultrasound pulse-echo technology, the hyperlens would allow the use of lower input frequency, which in turn would increase the penetration depth and allow physicians to see, for example, smaller tumors or finer features of larger objects that could help them identify other abnormalities,” Zhang says.
Acoustic hyperlens could be applied to underwater sonar as a focusing device that would allow more complex and precise custom waveforms to be created while still maintaining the power of the propagating source.
Berkeley researchers (from left) Guy Bartal, Xiaobo Yin, Lee Fok and Xiang Zhang shown with their acoustic hyperlens which boosts the magnification of sound-based imaging technologies such as ultrasound and underwater sonar by eightfold. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Experimental demonstration of an acoustic magnifying hyperlens
Acoustic metamaterials can manipulate sound waves in surprising ways, which include collimation, focusing, cloaking, sonic screening and extraordinary transmission. Recent theories suggested that imaging below the diffraction limit using passive elements can be realized by acoustic superlenses or magnifying hyperlenses. These could markedly enhance the capabilities in underwater sonar sensing, medical ultrasound imaging and non-destructive materials testing. However, these proposed approaches suffer narrow working frequency bands and significant resonance-induced loss, which hinders them from successful experimental realization. Here, we report the experimental demonstration of an acoustic hyperlens that magnifies subwavelength objects by gradually converting evanescent components into propagating waves. The fabricated acoustic hyperlens relies on straightforward cutoff-free propagation and achieves deep-subwavelength resolution with low loss over a broad frequency bandwidth.