University of Texas at Austin

Thursday, April 23, 2009

Light and air interacting

Conception of laser-generated filaments for lightning control.

Conception of laser-generated filaments for lightning control.

When something unexpected happened during Aaron Bernstein’s laser experiment in ionization, he veered from the original experiment, followed the surprise and found something he calls “pretty darn cool.”

What Bernstein, a scientist in the Department of Physics at The University of Texas at Austin, and his colleagues found was that they could cross two laser beams in ambient laboratory air and transfer seven percent of the energy of one of the beams to the other. They controlled the energy exchange by simply adjusting a time-delay between the two laser pulses.

The beams were intense enough to produce “optical filaments,” which are dramatic structures resulting from an interaction between the light and air. The filaments exhibit a focus of high intensity light that can persist for hundreds of meters of laser beam travel.

The same sort of interaction between light and air that leads to the filaments also enables the the energy flow from one beam to the other.

Such traveling high intensity laser beams are usually controlled by changing the pulses when the laser is launched. Bernstein’s innovation is a new “knob” used to control filaments during their actual flight.

The knob could be compared, crudely, to a dimmer switch on a light fixture. Turn the switch up and the light brightens. Turn it down and the light dims.

But, in Bernstein’s experiment, he used light mediated by air and not electricity flowing through wires, to control the laser beams.

“It can be thought of as an optical switch in the air or as a transistor,” he said. “So that’s pretty darn cool when you just have two beams of light going through the air and you’re able to control light with light.”

Controlling these filaments could lead to advances in lightning control and remote sensing.

To trigger lightning, beams would be shot into storms.

“[If we can] make the lightning strike when and where we want before it otherwise would have,” he said, “that would have obvious benefits for any installation of high value like the space shuttle or even oil pipelines.”

The development might help in “remote sensing” applications, useful for determining what is in air at a particular distance. An example would be what’s coming out of a factory smokestack, or whether there is poisonous gas over a battlefield.

Previous research on filaments had pointed out that it is hard to control where they turn on and off. But Bernstein has shown that this level of control might be possible.

“So now we’re not talking about changing launch conditions or optimizing launch conditions,” he said, “we’re talking about controlling the beam as it’s on its way. An exciting prospect.”

Bernstein’s paper about that experiment appeared in Physics Review Letters.

He said that being open to the unexpected is a key part of the research process.

“Some of going into the lab is just playtime, a little bit,” Bernstein said. “If I kept my quote-unquote “eye on the ball” I wouldn’t have stopped to look at this. But this was fun when we saw this.”

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