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The cloaking device created by the Duke team takes advantage of
this principle to bend light in precise ways. When light passes
from air into a denser medium such as glass or water, it slows
down and shifts direction or “refracts.”
“Light bends in glass [or water] because you’ve got
moving electrons in [the] atoms whose motions create their own
electromagnetic field,” explains Steven Cummer, an associate
professor of electrical and computer engineering at the Pratt School
who also worked on the invisibility device. Think of the classic
high-school lab experiment, in which a straw is dipped into a glass
cup half full of water. The submerged part of the straw looks as
if it is no longer continuous with the portion above the water.
The refraction of light as it passes from air into water creates
the illusion that the straw is broken.
The direction and degree that light rays bend when entering a new
medium is determined by an optical property called the index of
refraction. For glass, water, and most other natural materials,
the index of refraction is uniform throughout the entire material.
Not so with the Duke team’s metamaterial. “In our case,
the material properties vary from point to point in the cloak,” Schurig
says.
This means light can bend in many different directions at once
within the metamaterial. “The index of refraction varies
throughout our material, and it’s that variation that causes
the light rays to bend and go around the object,” Schurig
explains.
While scientific interest in metamaterials has skyrocketed in recent
years, their antecedents can be traced back to at least the time
of the Romans, says Ulf Leonhardt, a professor of theoretical physics
at the University of St. Andrews in Scotland. Roman craftsmen made
bright, blood-red glass, called “ruby” glass, by mixing
molten glass with microscopic spheres of gold. The diameter of
each gold sphere was thousands of times smaller than the width
of a human hair. At such a small scale, gold is no longer golden.
Electrons on the surface of the tiny particles absorb blue and
yellow light but reflect longer-wavelength red light. The result
is red, not gold, tinted glass.
Like modern metamaterials, ruby glass was made by combining two
or more natural materials to create a novel electromagnetic effect.
The difference between the Romans and modern scientists is that “we
now have more control,” Leonhardt says. “We can design
structures more clearly and compute them in advance, instead of
trying things out by trial and error. Our manufacturing processes
are also much better than what the Romans were able to do.”
The story of modern metamaterials is much younger and is still
being written, but it began with a group of scientists who figured
out how to bend light the wrong way. Until just a few years ago,
every material ever examined had a positive index of refraction,
meaning it always caused light passing through it to bend to the
right of the incoming beam. But in 1967 a Soviet physicist named
Victor Veselago showed that it was theoretically possible to create
a material with a negative index of refraction that could bend
light to the left.
Veselago further demonstrated that a negative-index material would
have startling and counterintuitive properties unlike anything
found in nature. Children swimming in a pool of negative-index
liquid, for example, would look as if they were doing backstrokes
in air because their reflections would appear above the pool’s
surface. And the aforementioned straw, if dipped into a glass of
negative-index water, would appear to bend all the way out of the
water in a V-shape.
For years, Veselago worked in vain to find or create a material
with the remarkable electromagnetic properties his formulas predicted.
But his efforts ended in failure and his idea eventually came to
be regarded by the physics community as a fascinating but far-fetched
possibility, the “unicorn” of electromagnetic research.
Indeed, negative-index research didn’t start to become serious
science again for some thirty years. In 2000, a team that included
Smith, then an associate adjunct professor of physics at the University
of California at San Diego (UCSD), created one of the world’s
first metamaterials, capable of doing exactly what Veselago had
predicted.
Smith’s team built upon the work of another scientist, Sir
John Pendry, a physicist now at Imperial College London. A few
years before, Pendry realized that a material could be thought
of as more than just a collection of identical atoms or molecules;
it could also be composed of larger multi-atom structures precisely
arranged in repeating patterns. Think of a valet-parked garage
where each vehicle lines up perfectly with the next to form a tidy
row-by-row pattern of cars. According to Cummer, Pendry’s
main insight was that scientists could fabricate whole structures
made up of multiple atoms that behaved like individual atoms in
a normal material.
A material made this way would be easier to manufacture because,
as Leonhardt puts it, “scientists can more easily make structures
than chemists can make new materials.” A metamaterial can
also be designed to respond to the electric and magnetic fields
that make up light in ways unknown in nature. (The prefix in “metamaterial” comes
from the Greek word “meta,” meaning “beyond.”)
Soon after Pendry had his epiphany, his team created two modern-day
metamaterials: One was an array of metal coils that manipulated
only the magnetic field component of light; the other was a lattice
of wires that affected only light’s electric field. “There
was some interest in that, but it didn’t become this huge
growing field until David Smith had the insight to put these two
things together,” Schurig says.
Smith’s team combined Pendry’s metal coils and wires
into a single metamaterial capable of manipulating both the electric
and magnetic field components of light simultaneously. In this
way, they were able to make Veselago’s fabled negative-index
material a reality. The metamaterial created by Smith’s team “showed
clearly that you could engineer something that was totally unlike
anything you could get out of nature,” Cummer says.
Schurig, a graduate student at UCSD at the time, remembers the
excitement—and skepticism—surrounding Smith and Schultz’s
success. “About half the scientists who had heard about it
didn’t believe it,” Schurig recalls. “But it
was interesting enough that a few more people started working on
it, and they were able to confirm those early results. More and
more people have been pouring into the field ever since.”
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