Light turns into glowing liquid
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 Frédéric 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."
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