Hiding objects with a terahertz invisibility cloak

ScienceDaily (Sep. 2, 2011) — Researchers at Northwestern University have created a new kind of cloaking material that can render objects invisible in the terahertz range. Though this design can't translate into an invisibility cloak for the visible spectrum, it could have implications in diagnostics, security, and communication.

The cloak, designed by Cheng Sun, assistant professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science, uses microfabricated gradient-index materials to manipulate the reflection and refraction of light.

Sun's research was published Sept. 1 in Scientific Reports, a new online, open-source journal that provides rapid publication and high visibility of research for all areas of science.

Humans generally recognize objects through two features: their shape and color. To render an object invisible, one must be able to manipulate light so that it will neither scatter at an object's surface nor be absorbed or reflected by it (the process which gives objects color).

In order to manipulate light in the terahertz frequency, which lies between infrared and microwaves, Sun and his group developed metamaterials: materials that are designed at the atomic level. Sun's tiny, prism-shaped cloaking structure, less than 10 millimeters long, was created using a technique called electronic transfer microstereolithography, where researchers use a data projector to project an image on a liquid polymer, then use light to transform the liquid layer into a thin solid layer. Each of the prism's 220 layers has tiny holes that are much smaller than terahertz wavelengths, which means they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism's bottom surface; the light then appears to be reflected by a flat surface.

Sun says the purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation.

"This demonstrates that we have the freedom to design materials that can change the refraction index," Sun said. "By doing this we can manipulate light propagation much more effectively."

The terahertz range has been historically ignored because the frequency is too high for electronics. But many organic compounds have a resonant frequency at the terahertz level, which means they could potentially be identified using a terahertz scanner. Sun's research into terahertz optics could have implications in biomedical research (safer detection of certain kinds of cancers) and security (using terahertz scanners at airports). Next Sun hopes to use what he's learned through the cloak to create its opposite: a terahertz lens. He has no immediate plans to extend his invisibility cloak to visible frequencies. "That is still far away," he said. "We're focusing on one frequency range, and such a cloak would have to work across the entire spectrum."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Northwestern University.

Journal Reference:

Fan Zhou, Yongjun Bao, Wei Cao, Colin T. Stuart, Jianqiang Gu, Weili Zhang, Cheng Sun. Hiding a Realistic Object Using a Broadband Terahertz Invisibility Cloak. Scientific Reports, 2011; DOI: 10.1038/srep00078

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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Proposed 'fluid flow cloak' might greatly reduce ships' hydrodynamic drag

 

A scientist has proposed a 'fluid flow cloak,' which might reduce the drag on ships' hulls by tricking the surrounding water into standing still

North Carolina's Duke University has been grabbing some headlines over the past few years, due to research carried out there involving the use of metamaterials for creating functioning invisibility cloaks. Just this month, Duke researchers announced that they had developed another such material that could be used to manipulate the frequency and direction of light at will, for use in optical switching. Now, Duke's Prof. Yaroslav Urzhumov has proposed that metamaterials could also be used to drastically reduce the drag on ships' hulls, "by tricking the surrounding water into staying still."

When you pull an object such as a fishing lure through the water, it tends to feel like it's much heavier than it actually is. This is because of the friction that occurs between the moving object and the stationary (or at least less quickly-moving) water around it. Not only does the object drag against the layer of water that's touching it, but to a lesser extent, it also has to pull along the water that's adjacent to that layer.

In Urzhumov's scenario, the layer of water touching the hull would be moving at the same speed as the ship, so it would be as if the water wasn't moving relative to the vessel - most of the friction would occur between that layer and the surrounding water, and not be focused on the ship. This would be made possible through a metamaterial coating the ship's hull, that was full of tiny holes and passages, kind of like a rigid sponge. Water would enter the material, then be ejected by small pumps, at a speed matching that of the water surrounding the vessel.

"I see this porous medium as a three-dimensional lattice, or array, of metallic plates," he said. "You can imagine a cubic lattice of wire-supported blades, which would have to be oriented properly to create drag and lift forces that depend on the flow direction."

Essentially, the moving ship would be surrounded by a cloak of still water. While some energy would be required to run the micro-pumps, Urzhumov believes that it would be more than made up for by the energy that the system would save.

A different system, recently proposed by the University of Melbourne's Prof. Derek Chan, would see ships made more hydrodynamic by enveloping their hulls in a layer of super-heated water.

The Duke University research was recently published in the journal Physical Review Letters.

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