Tie Jun Cui and colleagues at Southeast University in Nanjing, China used metamaterials to create a planar (flat) version of a Luneburg lens. Luneburg lenses are traditionally spherical optics that interact with light or RF in an unusual way--the lens bends the light more or less depending on where it strikes the lens. It’s able to do this because the index of refraction varies across the spherical lens body. The lens is able to focus light or incoming electromagnetic waves to an off-axis point at the edge of the lens instead of behind or in front of it. These devices can also be used in the reverse direction to channel waves from a nearby point source and radiate them in a single direction.
This characteristic makes the Luneburg lens useful as radar or microwave antennas, but its spherical shape has limited its use in some applications. By embedding an array of tiny, metallic U-shaped structures onto a dielectric material the Chinese researchers were able to create a new artificial surface that can bend and focus electromagnetic waves just as a spherical Luneburg lens does.
"We now have three systematical designing methods to manipulate the surface waves with inhomogeneous metasurfaces, the geometric optics, holographic optics, and transformation optics," said Cui. "These technologies can be combined to exploit more complicated applications."
For more information, see A broadband transformation-optics metasurface lens from AIP Applied Physics Letters. The article shows a diagram and photo of the 9 GHz test antenna and show patterns and performance at frequencies ±1 GHz. The illustrations make it easier to understand the complex mathematics behind this interesting antenna concept.