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Room temperature ruby slows light

28 Mar 2003

US researchers use a ruby to slow light to 57 meters per second at room temperature.

Using a technique known as spectral hole burning, US researchers have slowed light down in a ruby at room temperature. The researchers say their simple technique, which slowed the group velocity of light to 57.5 m/s, may now lead to controllable optical delay lines, which would find uses in telecommunications and computer networks. (Physical Review Letters 90 113903)

Being able to slow light down is nothing new. But up until now the phenomenon has only been seen at cryogenic temperatures and in exotic gases such as rubidium.

Robert Boyd and colleagues from the Institute of Optics at the University of Rochester use an argon-ion laser operating at 514.5 nm as their light source. The laser's output is then passed through an attenuator and an eletro-optic modulator to generate millisecond-long pulses.

The pulses are focused to an 84-micron-diameter waist on the ruby rod. According to the authors, the pulses had a peak power of 0.28 W, with a background that was only 4% of the peak.

The spectral hole forms due to coherent population oscillations within the ruby making it transparent to a narrow range of wavelengths. "The spectral hole can be extremely narrow, 36 Hz in our experiment," says the authors. This slows the pulses as they pass through the ruby.

The authors say the pulse slowing depends on the intensity and modulation of the light. "By moving the ruby a small distance away from the focus, we could greatly increase the measured delay," they report. "The group velocity can be controlled by changing the modulation frequency or the input intensity."

Because this technique is easy to implement and involves only one laser, the Rochester team reckons it could find practical applications in telecoms. Light pulses passed along transmission lines could be stopped or delayed in crystals to synchronize or store them.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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