A three-dimensional invisibility-cloaking structure operating at optical wavelengths based on transformation optics has been designed and realized. Our blueprint uses a woodpile photonic crystal with tailored polymer filling fraction to hide a bump in a gold reflector. Structures and controls are fabricated by direct laser writing and characterized by simultaneous high-numerical-aperture far-field optical microscopy and spectroscopy. Cloaking operation with large bandwidth of unpolarized light from 1.4- to 2.7-µm wavelength is demonstrated for viewing angles up to 60 degrees
Beyond military applications, cloaking devices are drawing interest from telecommunications companies, who see them as a way to send information by light more efficiently. One idea is to use the new materials to build “superantennas” that can concentrate light and other electromagnetic waves to make laser-like beams.
the cloak is a structure of crystals with air spaces in between, sort of like a woodpile, that bends light, hiding the bump in the gold later beneath, the researchers reported in Thursday’s online edition of the journal Science.
In this case, the bump was tiny, a mere 0.00004 inch high and 0.0005 inch across (100 microns x 30 microns), so that a magnifying lens was needed to see it.
“In principle, the cloak design is completely scalable; there is no limit to it,” Ergin said. But, he added, developing a cloak to hide something takes a long time, “so cloaking larger items with that technology is not really feasible.”
“Other fabrication techniques, though, might lead to larger cloaks,” he added in an interview via e-mail.
The value of the finding, Ergin said, “is that we learn more about the concepts of transformation optics, and that we have made a first step in producing 3-D structures in that field.”
Guardian UK – Tolga Ergin and Nicolas Stenger at the Karlsruhe Institute of Technology in Germany used a technique called direct laser writing lithography to create a sheet of cloaking material from tiny plastic rods. The spacing of the rods, each of which measured one thousandth of a millimetre wide, alters a property of the material known as the refractive index, which changes the speed of light inside it.
The researchers placed a piece of the material over a dimple in a gold sheet and used infrared cameras to see what happened. When the cloak was in place, it altered the speed of light around the bump in such a way that the gold sheet appeared to be flat. The experiment was equivalent to hiding something under a carpet and having the carpet disappear too.
It is the first time researchers have demonstrated a cloak that works in three dimensions. Previous devices have hidden objects when looked at head-on, but did not work if viewed from the side. “We were surprised that the cloaking effect was still so good, Ergin told the US journal, Science.
Inside the material, the plastic rods are arranged like planks of wood piled up on each other. The high precision of the structure means it is possible to control the refractive index so it varies in just the right way to bend light around whatever object is hidden beneath it.
“The material has a higher refractive index on top of the bump, so light hitting that part is slowed down a little bit compared with light impinging on the rest of the surface,” said Stenger. “That compensates for the shape of the bump, and in the end, it is exactly as if there was no bump.”
Research into cloaking devices has attracted funding from military organisations, such as the US Defence Advanced Research Projects Agency, which backs high-risk science research for the Pentagon. In the near term, cloaking materials are expected to be used to hide aircraft from radar more effectively.