Ultrasound and underwater sonar devices could "see" a big improvement
thanks to development of the world's first acoustic hyperlens. Created by researchers
with the U.S. Department of Energy's
Lawrence Berkeley National Laboratory (Berkeley Lab), the acoustic hyperlens
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.
The key to this success is the capturing of information contained in evanescent
waves, which carry far more details and higher resolution than propagating waves
but are typically bound to the vicinity of the source and decay much too quickly
to be captured by a conventional lens.
“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.”
In the world of optical imaging, hyperlensing is enjoying a hyper rage. Fabricated
from metamaterials - composites of metals and dielectrics whose uniquely engineered
structures give rise to extraordinary optical properties - hyperlenses make
it possible to overcome the so-called “diffraction limit” by imaging
features that are significantly smaller than the wavelengths of incident light.
Zhang called the capturing of information carried by evanescent waves “the
Holy Grail of optical information” in 2007, when he and his research group
announced a hyperlens made from nanowires of silver and aluminum oxide that
was able to use visible light to image objects smaller than 150 nanometers,
well below visible light's diffraction limit of 260 nanometers.
Sound waves are also hampered by an intrinsic diffraction limit when deployed
for imaging purposes - objects that can be seen with conventional acoustic imaging
are limited by the length of the sound wave. Once again, Zhang and his colleagues
have overcome this diffraction limit by employing carefully engineered wave
dispersion surfaces. This time they've demonstrated the first broad-band
low-loss imaging with large magnification, where evanescent waves carrying information
about subwavelength features are gradually converted into propagating waves.
“We provide a paradigm on the design and use of metamaterials to manipulate
sound waves down to subwavelength scales,” says co-author Li, a former
post doctoral fellow in Zhang's group and now an assistant professor in
City University of Hong Kong. “The success of our simple metamaterial
design opens further possibilities in manipulating sound waves, particularly
in transformation acoustics, which is analogous to transformation optics. Curved
coordinate mappings could also be used to design novel acoustic devices such
as a hyperlens with flat input and output facets.”
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,”
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.
Support for this research came from the Office of Naval Research.
Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research for DOE's
Office of Science and is managed by the University of California. Visit our
Website at www.lbl.gov/