http://www.newscientist.com/news/news.jsp?id=ns99992497
Light turns into glowing liquid
09:20 07 July 02 Exclusive from New Scientist Print Edition
Light can be turned into a glowing stream of liquid that splits into
droplets and splatters off surfaces just like water. The researchers
who've worked out how to do this say "liquid light" would be the ideal
lifeblood for optical computing, where chips send light around optical
"circuits" to process data.
The concentrated light pulse would bounce off a surface just like a
liquid drop
Liquid light sounds like a contradiction, since the three phases - gas,
liquid and solid - usually only apply to atomic matter. Although
researchers sometimes talk about a light beam as if it's a gas, because
the photons move around randomly within the beam and can exert pressure
due to their momentum, they don't usually mean it literally - until now.
You really can think of light as a gas, says Humberto Michinel's team at
the University of Vigo in Ourense. And like any gas, it can be made to
condense into a liquid.
The researchers have been working on "non-linear" materials, which slow
light down by an amount that depends on the intensity of the beam rather
than simply a fixed amount, as happens in water or glass. In most
non-linear materials, the more intense the light, the more it is slowed
down. That means the inside of a beam slows more than the outside, as if
it were passing through a convex lens, and the beam is focused to a
point, rather than transmitted as you'd want in an optical computer.
But this doesn't have to happen, Michinel realised. If you have a
material in which the light slows less when the intensity of the beam
gets very high, then a high-energy laser beam could be concentrated into
a tight column instead. This column behaves just like a liquid, says
group member Jose Ramon Salguiero at the University of Santiago de
Compostela in Spain.
Shattered drops
The researchers carried out computer simulations of what would happen to
a light pulse concentrated in this way. They showed that the pulse had a
kind of surface tension, making it stretchy on the outside, and that it
would shatter into smaller drops when it bounced off a surface, just like
a liquid.
Other researchers aren't convinced, however. "The name is catchy and it's
a clever idea, but I'm not sure it's really going to change things," says
Demetrios Christodoulides at Lehigh University in Bethlehem,
Pennsylvania, part of a competing team also working on non-linear
materials.
One problem is whether the material that Michinel wants to use to test
his predictions will be up to the task. Michinel reckons a "chalcogenide"
glass made by Fr�d�ric Smektala and colleagues at the University of
Rennes in France is just right for making liquid light. But
Christodoulides says that the material would have to interact so strongly
with the light that the droplets would probably be absorbed before they
could get anywhere.
But if the researchers can make liquid light, blobs of the stuff could
form the heart of an optical computer. The speed of silicon-based
processors is limited by the rate at which electrons move round circuits.
An optical computer based on photons would be much faster, but it's tough
to bounce light around without the beam spreading out and information
disappearing. "Liquid drops are optimal candidates to be information
bits," says Michinel.
But Christodoulides believes his own approach is a better bet: adjusting
the design of optical pathways so that they handle pulses of ordinary
light better. "Pulses are discrete things and you can do digital
operations with them," he says. "A liquid can end up anywhere and be
quite unpredictable."
