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Optical/IP

Scientists 'Stop' Light in a Solid

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
JStankus 12/4/2012 | 11:03:36 PM
re: Scientists 'Stop' Light in a Solid Is there a technical journal reference for this? It sounds very much like a photon echo, not "stopping Light". The photon echo technique is similar to the NMR pulse echo techniques developed in the 1960s. Photon echos have been around at least since the 70s, though 100sec sounds like a pretty long dephasing time.

John
dispersion 12/4/2012 | 11:03:33 PM
re: Scientists 'Stop' Light in a Solid Re: technical Journal Ref:

Phys. Rev. Lett. 88, 023602 (2002)
Pauline Rigby 12/4/2012 | 11:03:33 PM
re: Scientists 'Stop' Light in a Solid The reference for the paper is Phys. Rev. Lett. 88, 023602 (2002)

Here's a link to it, although you need a subscription to download the full text:

http://ojps.aip.org/getabs/ser...

In the paper it talks about trapped light storage being similar to Raman spin echo storage. Bit beyond me at that point.

[email protected]
JStankus 12/4/2012 | 11:03:32 PM
re: Scientists 'Stop' Light in a Solid Saw the ref after rereading after I posted. Thanks.

The question that this leaves is how long of an interaction length would be necessary to build a practical device? (Though liquid Helium cooling will never make a practical commercial device).

John
mattyl 12/4/2012 | 11:03:16 PM
re: Scientists 'Stop' Light in a Solid Try look at
http://rleweb.mit.edu/Publicat...
I think it's the same paper
turukhin 12/4/2012 | 11:03:07 PM
re: Scientists 'Stop' Light in a Solid Hi Folks,

Yes, it is a kind of similar to photon echo, and it is even more similar to spin echo...but still it is different. Normally, the efficiency of PE or SE can be 1-2%, here we can get almost 100% (I had 60% efficiency during the very first experiment). For any applications it makes a huge difference.
The interaction length was less than 3 mm.
Keep posting! :-)

Alex
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