ADVERTISEMENT

High-Intensity Lasers Throw Scientists a Curve

Researchers defy the laws of physics by making a laser beam bend
laser,plasma,lightning



© SCIENCE/AAAS

Ultra-intense lasers hold much promise for improving scientific tools such as laser-induced breakdown spectroscopy (LIBS), and deepening researchers' understanding of atomic, molecular, optical and plasma physics. The enormous intensity of these lasers (attributed to the brief but powerful pulses of energy they emit), however, makes it difficult for scientists to fully characterize and understand them.

Researchers at the University of Arizona in Tucson (U.A.) and the University of Central Florida in Orlando (U.C.F.) report in Science this week that they have found a way to bend a high-intensity pulsed laser beam, a breakthrough they are hoping will help them better understand how ultra-intense laser pulses travel through the air and find potential new uses for the technology.

"People expect lasers to do certain things, like propagate in a straight line," says lead researcher Pavel Polynkin, an associate research professor at U.A.'s College of Optical Sciences. "The fact that a laser beam actually curves is quite unusual."

Polynkin and his colleagues were the first to report bending the beam of a pulsed laser. But a U.C.F. team of scientists (including current study co-authors Demetri Christodoulides and Georgios Siviloglou) in November 2007 demonstrated a continuous wave (or steady stream) laser that curved slightly, turning on its ear the assumption that lasers can travel only in straight lines.

The U.C.F. researchers dubbed the set of waveforms making up this curved laser the "Airy" beam, after English mathematician and astronomer Sir George Biddell Airy , who in the 1820s first articulated the science behind rainbows.

Rather than use a steady-stream laser beam, Polynkin and his team used a high-intensity laser that emits short blasts of light, also called "light bullets," with each blast only 35 femtoseconds in duration. (A femtosecond is equal to one quadrillionth of a second.) Directly from the laser, these bullets are round (about 0.4 inch, or one centimeter, in diameter) and short (about 10 microns), corresponding to the ultrashort duration of the pulses. They resemble pennies, although much thinner and traveling at a speed of light. The researchers reshaped the profile of these pulses into that of an Airy beam using a thin plate of glass with a particular variation of thickness across the plate. "The phase shifts introduced by this plate turn the bullets from round in shape to the Airy beam that looks more like a triangle," Polynkin says.

Because the pulses have extremely high intensity, they ionize the air in their pathways, leaving a curved plasma stream in their wakes. Each bullet becomes an intense concentration of electromagnetic energy that travels along a curved trajectory and leaves a bent plasma channel behind. Overall, the self-bending beam does have its limits—the bullets do not deviate from a straight line by more than the beam's diameter. "If the beam is one centimeter [in diameter]," Polynkin says, "it won't curve more than one centimeter."

Although it may not seem like a dramatic curvature, the deviation is enough to enable scientists to measure, in detail, the distribution of the radiation produced by the bullets along their paths. When pulsed beams travel in a straight line, the radiation originating from different locations along the beam path overlap, and these overlapping patterns are difficult to observe.

"We don't really understand the [structure] of laser beams, which is very important," says study co-author Jerome Moloney, director of the U.A.'s Center for Mathematical Sciences. "The significance here is that you don't expect to see light change trajectory."

Once researchers know more about how ultra-intense laser pulses travel, they hope to put them to good use. One thought has been to shoot a pulsed laser into a cloud to draw out lightning in a storm and use the plasma channel formed in the laser's wake to guide the lightning away from homes and power lines. Another possibility: employ high-intensity lasers as remote illumination sources in spectroscopic studies of pollutants in Earth's upper atmosphere.

Rights & Permissions
Share this Article:

Comments

You must sign in or register as a ScientificAmerican.com member to submit a comment.
Scientific American Back To School

Back to School Sale!

12 Digital Issues + 4 Years of Archive Access just $19.99

Order Now >

X

Email this Article

X