[tt] NYT: Surpassing Nature, Scientists Bend Light Backward

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Surpassing Nature, Scientists Bend Light Backward
http://www.nytimes.com/2008/08/12/science/12ligh.html

By KENNETH CHANG

Using tiny wires and fishnet structures, researchers at the
University of California, Berkeley, have found new ways to bend
light backward, something that never occurs in nature.

This technology could lead to microscopes able to peer more deeply
and clearly into living cells. And the same kind of structures might
one day be adapted to bend light in other unnatural ways, creating a
Harry Potter-like invisibility cloak. "This is definitely a big step
toward that idea," said Jason Valentine, a graduate student and a
lead author of a paper to be published online Wednesday by the
journal Nature. But scientists are still far from designing and
manufacturing such a cloak.

The work involves materials that have a property known as negative
refraction, which means that they essentially bend light backward.
Once thought to be pure fantasies, these substances, called
metamaterials, have been constructed in recent years, and scientists
have shown they can bend long-wavelength microwaves.

Negative refractive materials can in principle lead to fantastical
illusions; someone looking down at a fish in a pool of negative
refractive liquid would see the fish swimming in the air above.

Two separate advances are described in two scientific papers being
published this week, one demonstrating negative refraction at
infrared and visible wavelengths. The second article will be
published in Friday's issue of the journal Science. Both papers come
out of the research laboratory of Xiang Zhang, a professor at the
Nanoscale Science and Engineering Center in Berkeley.

When a ray of light crosses the boundary from air to water, glass or
other transparent material, it bends, and the degree of bending is
determined by a property known as the index of refraction.
Transparent materials like glass, water and diamonds all have an
index of 1 or higher for visible light, meaning that when the light
enters, its path bends toward an imaginary line perpendicular to the
surface.

With the engineered metamaterials, scientists can create refractive
indices less than 1 or even negative. Light entering a material with
a negative index of refraction would take a sharp turn, almost as if
it had bounced off the imaginary perpendicular line.

In the Nature paper, the Berkeley researchers created a fishnet
structure with 21 layers, alternating between a metal and magnesium
fluoride, resulting in a metamaterial with a negative index of
refraction for infrared light. The researchers said by making the
fishnet structure even smaller, they should be able to do the same
with visible light.

In the Science paper, a different group of scientists in Dr. Zhang's
laboratory used a different approach, building an array of minuscule
upright wires, which changed the electric fields of passing light
waves. That structure was able to bend visible red light.

Dr. Zhang said both approaches had advantages and disadvantages.
"There are many roads to Rome," he said. "At this point, honestly
speaking, we don't know which road will be the best."

One application of negative index materials could be a "superlens."
Light is usually thought of as having undulating waves. But much
closer up, light is a much more jumbled mess, with the waves mixed
in with more complicated "evanescent waves."

The evanescent waves quickly dissipate as they travel, and thus are
usually not seen. A negative refraction lens actually amplifies the
evanescent waves, preserving detail lost in conventional optics, and
the hope is to eventually build an optical microscope that could
make out tiny biological structures like individual viruses.