Curving ‘Light Bullets’

Laser “light bullets” that can curve through the air might someday help scientists monitor air pollution, according to a new study.

The bullets are created by extremely short-duration, high-intensity laser pulses, lead author Pavel Polynkin, a physicist at the University of Arizona, told National Geographic Magazine.

A result of a numerical simulation, this image depicts a transverse profile of an ultraintense femtosecond-laser Airy beam after propagation in a nonlinear gaseous medium (air). The dominant lobe of the beam, in the corner of the beam pattern, propagates along a curved trajectory and leaves a bent plasma channel in its wake. The whisker-looking ghost beam results from a nonlinear beam reshaping on propagation. Images courtesy of Science/AAAS.

The pulses are so rapid that the beam is broader than it is wide – creating what Polynkin calls “pancakes” of light.

But the use of complex lasers that produce wave patterns called Airy beams causes the brightest part of the beam to bend as the pancake of light speeds away.

The superbrightness of the laser can also cause the pancake to change shape as it moves through air, Polynkin said.

“If the intensity exceeds a threshold, then the beam tends to self-focus – the pancake wants to become a very short needle.”

Within that needle, the light intensity gets so high that the air around it becomes electrically charged, briefly creating a conductive path of plasma.

Previous work suggested that such light bullets could be used to create human-induced lightning, which has implications for lightning control around sensitive structures such as tall buildings and airplanes.

Polynkin’s study suggests that when combined with the Airy beams, these plasma-producing lasers can also create curving “needle” bullets that might have other uses.

A photograph of a burn pattern on aluminum foil produced by an ultra-intense femtosecond-laser Airy beam after propagation in a nonlinear gaseous medium (air). The dominant lobe of the beam, in the corner of the beam pattern, propagates along a curved trajectory and leaves a bent plasma channel in its wake. The whisker-looking ghost beam results from a nonlinear beam reshaping on propagation.

However, the authors caution that their work does not mean we can build laser cannons that shoot at targets hidden behind walls.

That’s the first question everyone asks, Polynkin said, but “the answer is no.” The curvature of the beam is very small – too small for weapons applications.

Instead, the light pulses leave behind curving plasma trails that emit their own light, providing a way to monitor air pollution in the upper atmosphere without the need for airplanes or weather balloons, Polynkin told the magazine.

Shot into the sky, these light trails would illuminate the chemical signatures of atmospheric pollutants, which can then be recorded remotely.

In general, curved light pulses is an important development, said Jerome Kasparian, a physicist at the University of Geneva, Switzerland, who wrote an accompanying commentary.

“Up to now, it was only possible to bend a beam through the interaction with a medium” such as a lens, he said. “This in the first time you can have [a] laser that intrinsically propagates with a curve.”

This study appears in this week’s issue of the journal Science.

For more information, visit: www.sciencemag.org