The ground glass or plastic lenses that we are familiar with work by changing the path of light as it enters and emerges from the lens. However, the trajectory cannot be varied inside the lens itself and light continues through in a straight line. Such lenses suffer from geometrical and wave aberrations, so stacks of them are often used to improve performance. Nevertheless, distortion can occur when such corrective measures are employed.
Now, David R. Smith and his team have used metamaterials to design a lens with a varying index of refraction to help overcome the problem. In addition to a planar field of view and zero f-number, the new lens offers more than a full decade bandwidth, with no change in performance between 7 and 15 GHz.
Co-author Nathan Kundtz explains the principle behind the design. “The specific approach we used begins with a well known optic, the Luneburg lens. A Luneburg lens is spherical and focuses light from infinity to a point on the back of the lens. While this is a powerful optic, the spherical focal locus is a serious problem for standard charge coupled devices or antenna arrays, which are flat. By employing the same technique used in the design of metamaterial cloaks, we were able to change the shape of the Luneburg lens to make it amenable to a flat focal array. The resulting lens has focal properties which are nearly identical to those of the Luneburg, but has the potential to be used with real detector arrays.”
It is possible that metamaterial lenses could play a part in the future of telecommunications and radars where a wide field of view and high gain are needed, while at short wavelengths they could be used in optical devices that currently depend on an array of lenses.
“Devices based on this approach rely only on a gradient in the refractive index of the material and have been demonstrated at wavelengths as short as 1.5µm,” says Kundtz. He adds, “Large scale nano-patterning and novel material implementations have the potential to bring these structures to shorter and shorter wavelengths. There is good reason to hope that this will extend into the visible spectrum, but there's plenty of work to do in getting there!”