A year ago almost exactly, scientists created quite a stir when they claimed to have slowed down light and then "stopped" it dead in its tracks, a result that could have huge implications for building optical memories (see Storing Light).

The two independent groups of scientists involved -- from the Harvard-Smithsonian Center for Astrophysics and the Rowland Institute of Science -- both stopped light inside a vapor. But earlier this week, a different team claimed to be the first to repeat the experiment inside a solid material.

Alexey Turukhin and colleagues from the Massachusetts Institute of Technology (MIT), Texas A&M University, the Electronics and Telecommunications Research Institute (ETRI) in Korea, and the Air Force Research Laboratory, Hanscom published their results in the January 14 issue of the journal Physical Review Letters (PRL).

Being able to store light in a solid represents a big step forward, according to Philip Hemmer, one of the authors on the paper, who is now at Texas A&M. The previous work using vapors required cumbersome, expensive, ultra-high vacuum equipment, or fragile glass cells. For real applications, solid state devices are a must.

Of course, this is a very early-stage experiment requiring a lab full of equipment, so real applications won't be happening any time soon.

The solid used by Turukhin's team was – ready? – praesodymium-doped yttrium silicate (Pr:YSO), an optical crystal that can be bought commercially. Pr:YSO is one example of a rare-earth-doped insulator; erbium-doped fiber is another. It appears to be a stroke of luck that the type of material used to make optical fiber amplifiers is also the right sort of material for creating "slow light" effects.

The bad news is that "stopping" light in a solid is more complicated than doing it in a gas.

But before going into detail, let's recall the basics of the phenomenon. Put simply, there are two light beams shining on a cell. One, called the probe pulse, carries information, while the other, called the coupling beam, acts like a switch. When the probe pulse is inside the cell, the coupling laser is turned off to stop the light. At that moment, the information carried by the probe is transferred into the atoms inside the cell (specifically, it's stored in the spin state of the atoms). There it remains, until the coupling laser is turned back on, at which point, the original beam returns from the grave, Lazarus-like (but without the odor).

A couple of extra things have to happen to produce this effect in a solid, Hemmer explains. First, the solid material has to be prepared beforehand, by illuminating it with a laser of a third wavelength for a millisecond or so. And second, the solid needs advance warning before the light can be pulled out of storage. Like kids that fidget, the atoms in the solid cannot maintain the same spin state for very long. Hitting those atoms with a so-called "pi pulse" of light reverses the dephasing, allowing the original data to be recovered a short time later.

The scientists claim that their apparatus could be used to store light for up to 100 seconds, providing it is held at a frosty 5 degrees Kelvin (-268° C). However, they only stored light for a millisecond in their experiments.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com