Ruby Crystal Slows Light at Room Temperature

Brent D. Johnson

Over the past few years, researchers such as Lene Vestergaard Hau of Harvard University in Cambridge, Mass., have reported fantastic experiments in which they slow light to a crawl --and even stop it temporarily -- in ultracold clouds of atoms. Now a group at the University of Rochester has slowed light at room temperature to speeds regularly exceeded by race car drivers.

Twenty years ago, Robert W. Boyd, the M. Parker Givens professor of optics at the Rochester, N.Y., university, was performing experiments with laser spectroscopy when he discovered the phenomenon of spectral hole burning. Based on his observations, he predicted that it should be possible to saturate a velocity subgroup in a ruby, and he subsequently burned a 37-Hz hole in the absorption band of the crystal.

A team at the University of Rochester has slowed light to speeds of 127 mph in a ruby crystal at room temperature. Courtesy of Robert W. Boyd.

Boyd said that he left this discovery aside until last year, when, on a plane trip, he was pondering Hau's experiments with "slow light." He wondered if there might be another way to slow light without using Bose-Einstein condensates. "Then it hit me like a ton of bricks," he said. He had already done it -- or, at least, he had come very close -- back in 1983.

Returning to his original paper, he reconstructed the experimental setup. He and his colleagues Matthew S. Bigelow and Nick N. Lepeshkin focused the green output of a multimode argon-ion laser through an acousto-optic modulator into the ruby, partially saturating the chromium ions in the crystal. Then, using a probe laser, they sent light at a slightly different frequency through the ruby, producing offset frequencies that interacted. A silicon photodiode with a beamsplitter detected the speed of the pulse before and after the delay, and a storage oscilloscope measured the time domain.

By entangling photons in a quantum state called a coherent population oscillation, Boyd and his team have slowed light to the leisurely pace of 127 mph at room temperature. The next step is to experiment with other optical crystals and to measure the magnitude of the slowing in each. Once this is done, the scientists can investigate temporary optical storage devices that would function as optical buffers in slow-light optical systems.