Crystal slows and speeds light

By Kimberly Patch, 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 hour.

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 it."

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," he said.

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).

Timeline:   Unknown
Funding:   Government
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.


October 8/15, 2003

Page One

E-paper closes in on video

Magnetic memory makes logic

Old idea retooled for security

Crystal slows and speeds light

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