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Acoustic Cloaking Could Hide Objects from Sonar
The acoustic cloak Shu Zhang is developing harnesses the unique properties of metamaterials. Making metamaterials involves replacing the molecules of a material with man-made structures that can be viewed as "artificial atoms" on a scale that is much smaller than the relevant wavelength. The small size of these artificial atoms dramatically increases their ability to manipulate light and sound, which makes metamaterials particularly promising for use in microscope lenses, imaging, display technologies and computer chips.
In 2006, a group of researchers at Duke University showed how concentric rings of printed circuit boards could make objects invisible to microwave light. The rings created a "hole in space" around which microwaves were diverted much like water in a river flows around a rock. When a metal cylinder was placed at the center of the hole, microwaves flowed around the cylinder and emerged on the other side as if there were nothing there.
Zhang has created a numerical model to build a metamaterial cloak that similarly guides sound waves around objects in water. The model is based on the acoustic lumped circuit network. "The unit cell of the network is so small compared to the wavelength of sound that it becomes and effective anisotropic medium that guides sound flow around the cloaked object," Zhang said.
Computer simulations demonstrated that her numerical model successfully achieved a cloaking effect. Zhang's next step is to construct and test an actual physical version of the cloak based on that numerical model. Previous experimental tests of an acoustic metamaterial lens that Zhang and her adviser, Assistant Professor Nicholas Fang, created through a similar design approach demonstrated that the acoustic metamaterial focused sound with a negative refractive index. The lens was machined by carving an array of channels and holes into an aluminum sheet and then filling the channels with water. In theory, such a negative index superlens should focus so-called evanescent sound waves to beat the diffraction limit. The lens Zhang and Fang built achieved a resolution of about half the wavelength of the incident waves in a paper-not perfect, but among the best that has been achieved so far using purely passive focusing elements. The MechSE researchers detailed the results in a paper published May 15 in Physical Review Letters.
If their metamaterial cloak also works, considerably more work would need to be done before the cloak could be scaled up to hide a ship or a submarine. Zhang's mesh model is based on cloaking an object with a diameter of about .67 times the wavelength of light-a far cry from the 50-foot beam of a nuclear submarine.
Zhang works in the research group of Assistant Professor Nicholas Fang. She and other members of the group create devices for focusing photon and sound into nanometer scale. Such devices and technologies could lead revolutionary methods of diagnosing living cells at molecular scale details, without the use of an electron microscope, and open the door for the non-destructive screening of drugs and other biological materials. Inspired by recent scientific predictions of a series of artificial materials possessing negative index of refraction, Professor Fang's team aims to break the resolution limit set by light and sound waves because of their nature of diffraction. Using a thin and smooth silver film that resonantly transfer molecular scale details, Professor Fang and his group showed for the first time a sharp optical image with 30nm resolution, about 10 times better than the resolving power of state-of-the-art optical microscopes. Given that many proteins and enzymes are visible in the 50 nanometer range, the technique would allow scientists to view the transport of individual proteins and enzymes within living cells. His group is currently collaborating with Dr. Ling-Gang Wu, a leading neuron scientist from the National Institutes of Health to reveal the real-time dynamics of single synapse using optical nanoscopes.