Novel material takes 3D 'invisibility cloak' a step closer

Scientists in the United States reported a further step towards a celebrated "invisibility cloak" by masking a large, free-standing object in three dimensions. The lab work is the latest advance in a scientific frontier that uses novel materials to manipulate light, a trick that is of huge interest to the military in particular. Reporting in the New Journal of Physics, researchers at the University of Texas in Austin cloaked an 18-centimetre (7.2-inch) cylindrical tube from light in the microwave part of the energy spectrum. Those hoping for a Harry Potter-style touch of wizardry would be disappointed. To the human eye, which can only perceive light in higher frequencies, no invisibility would have been seen. But, say the researchers, the experiment is important proof of a principle that so-called plasmonic meta-materials can achieve a cloaking effect. A warplane cloaked with such materials could achieve "super-stealth" status by becoming invisible in all directions to radar microwaves, said co-lead investigator Andrea Alu. Plasmonic meta-materials are composites of metal and non-conductive synthetics made of nanometre-sized structures that are far smaller than the wavelength of the light that strikes them. As a result, when incoming photons hit the material, they excite currents that make the light waves scatter. The new experiment entailed making a shell of plasmonic meta-materials and placing the cylinder inside, and exposing the combination to microwaves. Microwaves scattered by the shell ran into microwaves bounced from the object, preventing them sending a return signal to the viewer. "When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency and invisibility at all angles of observations," said Alu. Any shape of object can be masked, he added. The cloaking worked best at a microwave frequency of 3.1 gigahertz, said the paper. The feat is a step forward because other techniques have entailed bending light around two-dimensional objects or, in 3D, masking microscopic bumps on mirrors or reflectors, an approach called "carpet cloaking," say the authors. The new concept could be modified for visible light, although any cloaked objects would be very small, in the micrometer range, as the plasmonic effect is linked to the wavelength of the light, Alu said in a phone interview. Even so, there could be important applications for microwave meta-materials, he said. "Camouflaging to radar is one important application, a super-stealth device to make objects invisible to radar," he said. "What we are thinking about is not necessarily cloaking the whole warplane but some hotspots, a part such as the tailplane that you would want to cloak because it reflects most of the energy (from microwave radar)." Another outlet would be in laboratories, filtering out the "backscatter" of light from the tip of high-powered optical microscopes. Unwanted light such as this impairs images of the object that is being scrutinised, and skews measurements.