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Topic Name: Negative Refraction of Visible Light
Category: Nanofabrication
Research persons: Prof. Harry Atwater (APh), Dr. Henri Lezec, Dr. Domenico Pacifici, Julie Biteen, Jennifer Dionne, Carrie Ross, Luke Sweatlock,
Collaborators: Prof. Albert Polman, Hans Mertens (AMOLF),Dr. Henri Lezec
Location: Thomas J. Watson Laboratory of Applied Physics,MS 128-95,California Institute of Technology,Pasadena, CA 91125, United States
Details
For the first time, physicists have devised a
way to make visible light travel in the opposite direction that it normally
bends when passing from one material to another, like from air through water or
glass. The phenomenon is known as negative refraction and could in principle be
used to construct optical microscopes for imaging things as small as molecules,
and even to create cloaking devices for rendering objects invisible.
In the March 22 in the online publication
Science Express, California Institute of Technology applied physics researchers
Henri Lezec, Jennifer Dionne, and Professor Harry Atwater, will report their
success in constructing a nanofabricated photonic material that creates a
negative index of refraction in the blue-green region of the visible spectrum.
Lezec is a visiting associate in Atwater's Caltech lab, and Dionne is a graduate
student in applied physics.
According to Lezec, the key to understanding
the technology is first in understanding how light normally bends when it passes
from one medium to another. If a pencil is placed in a glass of water at an
angle, for example, it appears to bend upward and outward if we look into the
water from a vantage point above the surface. This effect is due to the wave
nature of light and the normal tendency of different materials to disperse light
in different ways-in this case, the materials being the air outside the glass
and the water inside it.
However, physicists have thought that, if new
optical materials could be constructed at the nanoscale level in a certain way,
it might be possible to make the light bend at the same angle, but in the
opposite direction. In other words, the pencil angled into the water would
appear to bend backward as we looked at it.
The details are complicated, but have to do
with the speed of light through the material itself. Researchers in recent years
have created materials with negative diffraction for microwave and infrared
frequencies. These achievements have exploited the relatively long wavelengths
at those frequencies--the wavelength of microwaves being a few centimeters, and
that of infrared frequencies about the width of a human hair. Visible light,
because its wavelength is at microscopic dimensions--about one-hundredth the
width of a hair--has defeated this conventional approach.
Dionne, one of the lead authors, says that the
breakthrough is made possible by the Atwater lab's work on plasmonics, an
emerging field that "squeezes" light with specially designed materials to create
a wave known as a plasmon. In this case, the plasmons act in a manner somewhat
similar to a wave carrying ripples across the surface of a lake, carrying light
along the silver-coated surface of a silicon-nitride material, and then across a
nanoscale gold prism so that the light reenters the silicon-nitride layer with
negative refraction.
Thus, the process is not the same as the one
used for negative refraction of microwaves and infrared radiation, but it still
works, says Dionne. And this discovery is particularly exciting because visible
light, as its name suggests, is the wavelength associated with the world of
objects we see, provided they are not too small.
"Maybe you could create a superlens that can
beat the diffraction limit," says Dionne. "You might be able to see DNA and
protein molecules clearly just by looking at them, without having to use a more
complicated method like X-ray crystallography."
Atwater, who is the Howard Hughes Professor
and professor of applied physics and materials science at Caltech, says the
plasmonic technique indeed has potential for a compact "perfect lens" that could
have a huge number of biomedical and other technological applications. "Once the
light coming from a nearby object passes through the negative-refraction
material, it would be possible to recover all the spatial information," he says,
adding that the loss of this information is why there is ordinarily a limit to
the size of an object that can be seen in a microscope.
Even more tantalizing is the possibility of an
optical "invisibility cloak" device that would surround an object and bend light
in such a way that it would be perfectly refocused on the opposite side. This
would provide perfect invisibility for the object inside the cloak, in a manner
similar to the cloaks used by Harry Potter or the Klingons in the old Star Trek
television series.
"Of course, anyone inside the cloak would not
be able to see out," Atwater says.
"But maybe you could have some small windows,"
Dionne adds.
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