Liquid light bender proposed

Tiny nanoparticles dispersed in fluid may hide objects

Tiny silver-coated rust particles suspended in water may give the fluid light-bending superpowers, physicists suggest in a paper to appear in Physical Review Letters.

Simulations with the proposed fluid system find that it could disguise objects from many wavelengths of visible light, lead author Jiping Huang of Fudan University in Shanghai and colleagues report. What’s more, the system would be tunable, giving researchers control over the light-contorting particles.

The ability to twist and contort light in unusual ways has been demonstrated in a special class of materials called metamaterials. New metamaterial designs may lead to optical devices such as cloaks and powerful lenses (SN: 11/21/09, p. 18).

The proposed metamaterial would be made up of silver-coated spherical magnetite nanoparticles, each about 10 nanometers wide, floating freely in liquid water. Under everyday conditions, the nanoparticles should float innocuously, causing water to behave normally. But when a magnetic field is applied, the particles will link up to form chains and then thicker columns as the chains attract each other. In the researchers’ simulations, these chains and columns of the metamaterial were able to bend light more than halfway back on its original direction. Light bending in the opposite direction is a phenomenon called negative refraction.

“It is fascinating that fluids can exhibit as unusual optical properties as negative refraction,” comments cloaking expert Ulf Leonhardt of the University of St. Andrews in Scotland.

Unlike in solid systems, researchers could control the properties of the proposed liquid-based system with an external magnetic field. Such tunability means that a single liquid metamaterial can bend light differently under different conditions, Huang says.

Researchers have already created a cloak that bends waves in the microwave frequency. With the right nanoparticles under the right magnetic conditions, the proposed fluid system could bend waves at many frequencies. “In principle the system can work at the entire visible spectrum as long as one chooses the right coating,” Huang says.

What’s more, any liquid in which the nanoparticles can evenly disperse and move unencumbered would work, Huang says. The new simulations show how the metamaterial would perform in water, but the work suggests the particles would behave similarly in a fluid such as kerosene, the authors write.

Study coauthor Xiang Zhang, of the University of California, Berkeley and Lawrence Berkeley National Laboratory, is currently working on creating the system in the lab, Huang says. The real challenge is not building the system, Huang says, but making it useful for specific devices, such as lenses and cloaks.

Leonhardt notes that the system has some drawbacks to overcome before it could work well. For instance, he says, the fluid metamaterial can twist light that comes in from only one orientation. Therefore it would not be able to manipulate natural light, which is a mish-mash of many orientations. “I am not sure whether there is an immediate practical application, but it is cool anyway.”

    Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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