PORTLAND, Ore. The pursuit of so-called invisibility cloaks has spawned a new field called transformation optics that seeks to harness exotic metamaterials, nanophotonics and plasmonics to build super lenses for powerful microscopes.
Transfomation optics were used recently to develop an improved invisibility cloak with a 100-fold increase in area compared to previous visible-wavelength cloaks. The new design from Purdue University uses a relatively inexpensive glass and gold waveguide that achieved a more economical design using transformation optics.
"When you send light down a waveguide and taper it properly, you can make that light bend around a certain area," said Purdue professor Vladimir Shalaev. "The light's phase velocity speeds up as it bends around the cloaked object, so that when it reaches the other side it has the same phase as if it had propagated through air."
|An inexpensive waveguide directs light around cloaked objects (center) so that even a laser bends around it to emerge on the other side with no shadow cast.|
Other invisibility cloaks use optical diffraction gratings that tune into specific wavelengths with a negative index of refraction that allows the cloak to provide invisibility, but only at those wavelengths. The Purdue device was formed from two gold-coated surfaces, one a curved lens, the other a flat sheet. The result was a broadband cloak, working at a wide swath of wavelengths simultaneously, enabling it to shield an area covering the entire spectrum of visible light.
The demonstration showed how a laser benda around the cloaked area, leaving no shadow, even though the area was 100 times larger than the wavelength of the laser.
Recently, metamaterials used in fields of split-ring resonators have been demonstrated for microwave and sonar frequencies. This and other intricate structures, however, would be much more expensive to manufacture than Purdue's relatively simple gold and glass waveguides.
The Purdue researchers claim that their waveguide cloaks the largest area yet at visible wavelengths, despite the fact that it was just 60 microns in diameter. They also predict that their scheme can be scaled up to cloak larger areas, adding that they are more interested in applying their current light-bending technology to other transformation optics appliccations such as better microscope lenses.
The ability to bend light in the same way as a metamaterial could enable correction lenses that probe dimensions smaller than conventional lenses.
Metamaterials could enable super lenses by varying their index of refraction below one, according to Shalaev. Natural materials have an index of refraction of greater than one. Shalaev's group claims that their waveguide can vary the index of refraction anywhere between zero and one. By tapering the waveguide properly, the index of refraction could be changed, allowing the gold and glass structures to provide near-perfect lenses and cloaks.