Crystal slows and speeds light
Technology Research News
Researchers from the University of Rochester
have tapped a 40-year old concept to show that it is possible to both
slow and speed light as it travels through a certain type of crystal.
The work expands on research that proved it was possible to slow
light to a stop, store its properties in atoms, then reconstitute the
light. Slowing and stopping light could be useful for communications,
data storage, and quantum computing and communications.
The researchers used oscillations within tightly-focused lightwaves
to trigger narrow spectral line shapes -- dips or peaks in the spectrum
of lightwaves a crystal's atoms absorb. The effect caused the refractive
index of the crystal to change rapidly as a function of the light's frequency,
said Matthew Bigelow, a researcher at the University of Rochester.
The refractive index dictates the angle light bends as it passes
from one material to another, and is responsible for the illusion that
a straight drinking straw in a glass of water bends at the water line.
The narrow spectral line shape concept was developed 40 years
ago, said Bigelow. "Narrow spectral line shapes... have been well-known
for a long time, but no one had thought to use them to create slow or
fast light," he said.
The researchers shot a pair of laser beams through a piece of
alexandrite crystal, and created narrow spectral line shapes using the
interactions between the two lasers.
When the researchers caused the refractive index of a piece of
alexandrite crystal to decrease rapidly, light traversed the crystal 800
meters per second faster than it normally travels through this type of
crystal, said Bigelow.
And when they caused the refractive index to increase rapidly,
light going through the crystal slowed down to just 91 meters per second,
said Bigelow. This works out to 3.4 miles per second, or 204 miles per
The researchers were able to switch between slow and fast, or
superluminal, light simply by switching the wavelength of one of the lasers.
The researchers were expecting to see superluminal effects in
the alexandrite crystal, "but we did not expect to see slow light there
as well," said Bigelow. Once they saw the phenomenon, they realized it
had to do with the location of the chromium ions that were affected, or
excited, by the laser, he said.
Chromium ions inhabit two different types of sites within the
lattice structure of the crystal: mirror sites and inversion sites. "At
a mirror site, the atoms in front of the ion would look like a mirror
image of the atoms behind it," said Bigelow. "At an inversion site, the
atoms in front of the ion look like an inverted image of [those] behind
When the chromium ions in mirror sites were excited, light traveled
through the crystal faster than usual. When the excited chromium ions
were located in inversion sites, it traveled slower than usual.
In contrast to previous experiments that changed the speed light
travels through a material, the Rochester method is relatively simple,
said Bigelow. Previous methods require super-cooled or superheated materials
and lasers turned to emit only a very narrow wavelength of light. "This
work is... relatively easy and simple to implement, [and] we can see this
effect over a relatively large frequency region -- tens of nanometers,"
None of these experiments change Einstein's basic concept of relativity,
which says that light's top speed when uninhibited by matter is 186,000
miles per second. The speed of light changes depending on what the light
is traveling through. Light travels through air at about 166,000 miles
per second, water at about 125,000 miles per second, and glass at about
110,000 miles per second.
The experiment is well done, and it agrees approximately with
existing theory, said Philip Hemmer, an associate professor of physics
at Texas A&M University.
The experiment is novel because the researchers showed it is possible
to produce slow or fast light independently of the optical coherence time
of the atomic system, according to Hemmer. Optical coherence time is related
to the spread of wavelengths within a laser beam; unlike previous experiments,
the researchers' method isn't limited to lasers that have narrow coherence
times. They "used an atomic system, yet the light speed was not determined
by the optical coherence time as in previous slow and fast light experiments
[that were] based on atomic systems," he said.
The current method is not really suitable for any practical applications,
but opens up new territory that may eventually lead to new ways of doing
things, said Hemmer. "Its main contribution is it forces researchers to
consider a wider class of physical effects that can be used to produce
slow or fast light," he said. "It is likely that from this wider class
a novel room temperature scheme can be devised that does perform well
enough to be useful for high-profile applications such as optical delay
lines," he said.
The researchers are working on increasing the bandwidth of the
signal transmitted through the system, said Bigelow. "We are currently
looking for solid-state materials with higher bandwidth suitable for communication
applications," he said.
The slow light phenomenon may eventually be used to add controlled
delays to optical communications equipment, said Bigelow.
It is difficult to predict when the phenomenon could be used in
practical applications, however, Bigelow added. "We need to increase the
bandwidth first before we can talk about when direct technological applications
will be available," he said.
Bigelow's research colleagues were Nick N. Lepeshkin and Robert
W. Boyd. The work appeared in the July 11, 2003 issue of Science.
The research was funded by the Department of Energy (DOE), the Army Research
Office, and the Air Force Office of Scientific Research (AFOSR).
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Quantum Computing and Communications; Physics
Story Type: News
Related Elements: Technical paper, "Super Luminal and Slow
Light Propagation in a Room-Temperature Solid," Science, July 11, 2003.
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