Portland, Ore. - Metamaterials that are able to reverse basic optical properties of conventional lenses and microwave antennas are being explored as a superior optical medium.
Normal materials refract electromagnetic radiation by bending it away from the angle of incidence, which requires that lenses be convex in order to focus. Left-handed metamaterials, on the other hand, bend light toward the angle of incidence, thereby enabling a planar lens to focus radiation to a point.
"Using left-handed metamaterials, we can build novel, smaller, lighter-weight lenses, sensors and antenna systems than those that are available today," said Srinivas Sridhar, a professor at Northeastern University. "Besides cheaper and better, our form factor is also more flexible, because metamaterials can conform to odd shapes since they are composed mostly of air. In our experiment, we used a periodic array of aluminum-oxide rods in air laid out in a lattice like a photonic crystal."
Sridhar performed the work with research associates Patanjali Parimi and Wentao Luj as well as doctoral candidate Plarenta Vodo.
The "meta" in metamaterials means they substitute macroscopic objects-rods in this case-for atoms in a macrosized, crystalline-like lattice. The pitch of the lattice's grid sets the wavelength affected. Unlike normal lenses, that wavelength can be set to an arbitrarily small subwavelength, giving the lens a nearly infinite focusing and resolution capability.
"Our research experimentally demon-strates for the first time the feasibility of designing a broadband left-handed lens," said Sridhar.
The novel thing about Sridhar's current demonstration is that the back edge of his photonic lattice is shaped like a concave cavity. A flat metamaterial lens will focus inward automatically, but Sridhar's practically hemispherical back edge bends the focal point of the lens even further inward, enabling it to focus images of far-off objects within the cavity.
"For near-field focusing, a flat lens is fine, but for focusing far-off objects like the stars, you need a concave lens when using a metamaterial, not the normal, convex lens," said Sridhar.
The research group also reported on an experiment, funded by the U.S. Air Force, in which a point source of microwaves was placed at the focal point of the lens and transmitted through the photonic crystal. The result was a parallel wavefront of microwaves that emerged from every point of the flat side of the left-handed lens. Such an outcome shows that a metamaterial could be used in the future to enable a new kind of directional antenna that radiates all along its flat surface.
The researchers also are working on new types of left-handed materials. The group's current plano-concave lenses focus microwaves with a periodic array of aluminum-oxide rods that have an 8.9 dielectric constant. The lattice spacing ratio was .175, and the radii of curvature of the lenses were 13.5, 17.5 and 22 centimeters. By adjusting those parameters while downsizing to closer spacings, the researchers hope eventually to achieve photonic lattices that work at all frequencies of interest.
"Many applications require far-field imaging of lenses in optics, astronomical telescopes, and commercial and defense microwave communications. Negative refraction allows focusing with reduced aberration to produce sharper images with enhanced resolution," said Sridhar.