PORTLAND, Ore. What is claimed as the world's first metamaterial for visible wavelengths was demonstrated recently at the U.S. Energy Department's Ames Laboratory.
Metamaterials exhibit a negative index of refraction, thereby enabling a flat lens to precisely focus light in place of a concave lens. Such metamaterials may also someday enable applications that cloak objects to make make them invisible.
"This is a big step forward," said Ames Laboratory senior physicist Costas Soukoulis. "We have for the first time, with our collaborators in Germany, created a negative index material at optical wavelengths." Soukoulis said the researchers "fishnet design" worked well at gigahertz and terahertz frequencies, adding that "next we want to improve our material before we try it in applications."
The Ames researchers worked with Stefan Linden and Martin Wegener at the University of Karlsruhe, Germany.
Metamaterials substitute macroscopic objects for atoms in a giant, crystalline-like lattice with a pitch smaller than the wavelengths passing through them. Metamaterials also can exhibit a negative permittivity and permeability, thereby enabling a negative index of refraction. That attribute enables metamaterials without an optical axis to focus waves, despite their planarity, by means of refraction.
Metamaterials were first hypothesized in 1968 by Russian theorist Victor Veselago, who theorized that light would interact with metamaterials in a way fundamentally different from natural materials, which have positive permeability and permittivity.
Numerous researchers have proven that metamaterials exhibit a negative index of refraction, but only for relatively long wavelenghths such as microwaves and more recently, the near infrared. Various researcher have fabricated metamaterials made from submicron metal rings and rods, therby enabling exquisite focusing on nanoscale features.
Until now, none exhibited a negative index of refraction for visible light.
The Ames researchers achieved that goal by using semiconductor fabrication equipment to create a chip with a fishnet-like mesh of 100-nm holes in a silver substrate, yeilding a refractive index of -.6 at the barely visible wavelength of 780 nm (deep red). Since this was the first demonstration of a metamaterial at visible wavelengths, Soukoulis cautioned that years of work will be needed to make the material efficient for real-world applications.
The group's next step will be verifying its preliminary results, then introducing active-amplifying materials into the mix to counter current signal losses.