Curved light bends the rules

Everyone knows that light travels in a straight line — right? A couple of years back, however, physicists discovered something very different for certain laser pulses that have one intense peak next to a series of smaller peaks. The brightest part of these lopsided "Airy" pulses, they found, appear to follow a curved trajectory.

Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.

The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.

The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.

Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.

The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.

As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.

Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.

With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.

Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.

Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.

Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.

The team are now studying the creation of curved filaments in water rather than air.

Laser triggered lightning

Reported from Photonics.com: Scientists have used ultrashort laser pulses to trigger electrical activity in thunderclouds, a first step toward creating man-made lightning.

In a modern-day take on Benjamin Franklin's experiment during a storm more than 200 years ago with a kite, a key and a silk ribbon to prove electricity exists in the atmosphere, the French, Swiss and German scientists aimed high-power pulses of laser light into two passing thunderstorms at the top of South Baldy Peak in New Mexico. The laser pulses created plasma filaments that could conduct electricity. No air-to-ground lightning was triggered because the plasma filaments were too short-lived, but the laser pulses generated discharges in the thunderclouds themselves, the scientists said.

Triggering lightning strikes is an important tool for basic and applied research because it enables researchers to study the mechanisms underlying lightning strikes. Triggered lightning strikes will also allow engineers to evaluate and test the lightning sensitivity of airplanes and critical infrastructure such as power lines.

The idea of using lasers to trigger lightning strikes was first suggested more than 30 years ago, but until recently lasers were not powerful enough to generate the long plasma channels needed. The current generation of more powerful pulsed lasers, like the one developed by Kasparian's team, may change that because they can form a large number of plasma filaments -- ionized channels of molecules in the air that act like conducting wires extending into the thundercloud.

Kasparian and his colleagues involved in the Teramobile project, an international program initiated by the National Center for Scientific Research (CNRS) in France and the German Research Foundation (DFG), built a powerful mobile femtosecond-terawatt laser capable of generating long plasma channels by firing ultrashort laser pulses. They chose to test their laser at the Langmuir Laboratory in New Mexico, which is equipped to measure atmospheric electrical discharges. Sitting at the top of 10,500-ft South Baldy Peak, this laboratory is in an ideal location because its altitude places it close to the high thunderclouds.

During the tests, the research team quantified the electrical activity in the clouds after discharging laser pulses. Statistical analysis showed that their laser pulses indeed enhanced the electrical activity in the thundercloud where it was aimed—in effect they generated small local discharges located at the position of the plasma channels.

The limitation of the experiment, though, was that they could not generate plasma channels that lived long enough to conduct lightning all the way to the ground. The plasma channels dissipated before the lightning could travel more than a few meters along them. The team is currently looking to increase the power of the laser pulses by a factor of 10 and use bursts of pulses to generate the plasmas much more efficiently.

The paper, "Electric Events Synchronized With Laser Filaments in Thunderclouds," appears in the April 14 issue of Optics Express, the Optical Society of America's (OSA) open-access journal.