DURHAM, N.C. -- Duke University engineers have created a new
generation of lens that could greatly improve the capabilities of
telecommunications or radar systems to provide a wide field of view
and greater detail.
But the lens they fashioned doesn't look anything like a lens.
While traditional lenses are made of clear substances – like
glass or plastic – with highly polished surfaces, the new
lens looks more like a miniature set of tan Venetian blinds. Yet
its ability to focus the direction of electromagnetic rays passing
through it dramatically surpasses that of a conventional lens, the
engineers say.
The latest advance was made possible by the ability to fabricate
exotic composite materials known as metamaterials. The metamaterial
in these experiments is not so much a single substance, but the
entire man-made structure which can be engineered to exhibit
properties not readily found in nature.
The prototype lens, which measures four inches by five inches
and less than an inch high, is made up of more than 1,000
individual pieces of the same fiberglass material used in circuit
boards and is etched with copper. It is the precise arrangement of
these pieces in parallel rows, that directs the rays as they pass
through.
"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 Kundtz, post-doctoral 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," Kundtz said.
"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."
The results of his experiments, which were conducted in the
laboratory of senior researcher David R. Smith, the William Bevan
Professor of Electrical and Computer Engineering, appeared as an
advanced online publication of the journal Nature Materials.
This is the first demonstration of what was thought to be
theoretically possible.
Recognizing the limitations of traditional lenses, scientists
have long been investigating other options, including those known
as gradient index (GRIN) lenses. These are typically clear spheres,
and while they have advantages over traditional lenses, they are
difficult to fabricate and the focus point is spherical.
Additionally, because most sensing systems are oriented in two
dimensions, the spherical image doesn't always translate clearly on
a flat surface.
The new lens, however, has a wide angle of view, almost 180
degrees, and because its focal point is flat, it can be used with
standard imaging technologies. The latest experiments were
conducted with microwaves, and the researchers say it is
theoretically possible to design lenses for wider frequencies.
"We've come up with what is in essence GRIN on steroids," said
Smith, whose team used similar metamaterials to create one of the
first "cloaking" devices in 2006. "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," Smith said. "The properties of the
metamaterials we used should also make it possible to use infrared
and optical frequencies."
The researchers say a single metamaterial lens could replace
traditional optical systems requiring vast arrays of lenses and
provide clearer images. They could also be used in large-scale
systems such as radar arrays to better direct beams, a task not
possible for traditional lenses, which would need to be too large
to be practical.
SOURCE