Researchers have made a breakthrough in the search for a cloaking device that can make objects appear invisible to the external observer, an area of enquiry that is attracting a lot of scientific interest. Although it only works at the nanoscale at the moment, potential future applications include important uses in military and communications technology.
The team of nanotechnologists, from Cornell University’s School of Electrical and Computer Engineering, have been working on silicon nanophotonics for several years, before recently realizing that the material system that they had been using is ideal for this cloaking application. The device works by manipulating the direction and behavior of light, to give an effect of invisibility, both for the distortion and the cloak that hides it.
Published online in Nature Photonics (DOI: 10.1038/NPHOTON.2009.117), the research by Lucas Gabrielli, Jaime Cardenas, Carl Poitras and Michal Lipson is based on a design by British physicists Ulf Leonhardt and Sir John Pendry, and demonstrates a cloak operating in the near infrared at a wavelength of 1,550 nm. This cloaking conceals a deformation on a flat reflecting surface, under which an object can be hidden. The cloak acts to bend light off the surface so as to correct for the distortion on the surface.
The research team made a reflector of about 30 microns long with a bump in the middle of only about 5 microns width, before locating a range of vertical silicon posts in front of the bump. With each posts being smaller than the wavelength of the light, the light acts as if it were passing through a solid – and, depending on the density of these posts, the light that passes through the posts can be controlled to compensate for the distortion that the bump makes.
Using transformational optics, this innovation can be applied at optical frequencies for the first time, which opens up a whole range of potential functions, including lenses and other type of optical devices, such as splitters. As Lipson explains, the team are "now using transformational optics to show perfect lenses – ie, lenses that concentrate well below the diffraction limit", which could bring about whole new areas for their research.
On a more short-term basis, cloaking could also help alleviate, for example, defects in lithographically fabricated chips. By "concealing" the defect, the mask used for fabricating chips could be a useful advance.