‘GRIN on steroids’: New lens uses opaque metamaterials

Laura S. Marshall, laura.marshall@laurin.com

It doesn’t look like a lens; it looks more like a tiny set of Venetian blinds. It’s not even made of traditional lens materials such as highly polished glass or plastic. But a new generation of lenses has the potential to make big changes in radar and telecom systems by offering greater detail and a wide field of view, according to engineers from Duke University.

Advances in metamaterials allow the new lens to focus the direction of electromagnetic rays passing through it in a way far superior to that of conventional lenses. More than 1000 pieces of the same fiberglass material used in circuit boards make up this prototype lens, which is etched with copper, measures 4 × 5 in., and is less than 1 in. high. Arranging the pieces in precise parallel rows enables the lens to direct rays as desired.

“For hundreds of years, lens makers have ground the surfaces of a uniform material in such a way as to sculpt the rays as they pass through the surfaces,” said Nathan B. Kundtz, postdoctoral associate in electrical and computer engineering at Duke’s Pratt School of Engineering. “While these lenses can focus rays extremely efficiently, they have limitations based on what happens to the rays as they pass through the volume of the lens.

“Instead of using the surfaces of the lens to control rays, we studied altering the material between the surfaces. If you can control the volume, or bulk, of the lens, you gain much more freedom and control to design a lens to meet specific needs.”

A new lens is a carefully constructed lattice of circuit board materials. Courtesy of David R. Smith Lab, Pratt Engineering. Credit: Duke University Photography.

Clear spheres known as gradient index (GRIN) lenses have been investigated as an alternative to conventional lenses, but they are difficult to fabricate. Also, because they have spherical focus points, they have proved difficult to incorporate into most sensing systems, which are two-dimensional and have trouble processing the spherical images. The new lens has an angle of view of almost 180° and a flat focal point.

Kundtz conducted his experiments in the laboratory of senior researcher David R. Smith, the William Bevan professor of electrical and computer engineering, and provided the first demonstration of what previously had only been theorized. He used microwaves for his most recent experiments, noting that it is theoretically possible to design lenses for wider frequencies as well.

“We’ve come up with what is in essence GRIN on steroids,” Smith said. “This first in a new class of lenses offers tantalizing possibilities and opens a whole new application for metamaterials.

“While these experiments were conducted in two dimensions, the design should provide a good initial step in developing a three-dimensional lens. The properties of the metamaterials we used should also make it possible to use infrared and optical frequencies.”

With metamaterials similar to those used in the new lens, Smith’s team created one of the first “cloaking” devices in 2006.

According to the researchers, a single metamaterial lens could be used to replace traditional optical systems that require vast arrays of lenses; the new lenses also could provide clearer images. Although size constraints keep traditional lenses from being practical for large-scale systems such as radar arrays, this new generation of lenses could be used to better direct beams in such systems, they say.

The research was supported by the US Army Research Office’s Multiple University Research Initiative. The results appeared as an advance online publication of the journal Nature Materials.