An Airy Beam viewed in cross section. The asymmetric beam has an intense region on the right-hand side and several less intense spots on the left (Image: Science)

Lasers may have thousands of applications in every section of modern society, but all laser beams are fundamentally similar – single-coloured and straight.

Now, US physicists have helped to break that mould by creating the first curved laser beams. The feat could one day help guide lightning to the ground.

Optics researchers led by Pavel Polynkin at the University of Arizona in Tucson generated 35-femtosecond-long laser pulses from a standard titanium-sapphire system.


The straight laser pulses differ from standard lasers in that they cover a wide range of colour frequencies rather than a single colour. Each pulse then passes through a transparent “phase pattern” mask and a lens, which together divide the laser pulse into its constituent parts, rather like breaking a musical chord into its individual notes.

The intensity profile of a normal laser beam is symmetrical around a central intense region, but the mask and lens are specially designed to impose an unusual asymmetric intensity profile, creating a so-called Airy beam. On the right-hand side of the Airy beam there is one intense bright region, and there is a series of smaller, less intense regions to the left (see image).

Plasma arc

As the pulse moves away from the lens, energy flows between the different intensity regions. Because of the underlying asymmetry, the beam bends around 5 millimetres to the right over its 60-centimetre measured length. That’s not enough to send the beams around sharp corners, but they could be guided around objects, such as cells, in microscopic applications.

There is such an intense concentration of electromagnetic energy in the strong curving peak of the pulse that it ionises the air as it travels, leaving a plasma arc in its wake. Team member Demetrios Christodoulides of the University of Central Florida in Orlando says those plasma arcs can help in analytical procedures.

“The emissions generated during the process are indicative of the gas composition [that the plasma is travelling through],” he says.

Spectacular application

Previously, straight plasma channels have been used to produce those emissions, but all the emissions are projected forwards onto the same spot. Since the forward direction constantly changes for the arc, however, the emissions would arrive at different points on a detector. “Now, because the emissions are from a curved plasma pattern, you can pinpoint precisely where in the gas they came from,” Christodoulides says.

But Jérôme Kasparian at the University of Geneva in Switzerland, who was not involved in the study, thinks plasma channels produced by laser pulses could perform a more spectacular function on a larger scale.

Kasparian and Jean-Pierre Wolf, also at Geneva, are attempting to use plasma channels to control lightning strikes by firing laser pulses into thunderclouds.

Curved lightning

In 2004, Kasparian and Wolf took their plasma channel-generating laser equipment into thunderstorms in New Mexico. They fired straight laser pulses into the thunderclouds 10 times every second, hoping that the high energy plasma channels that form in the laser pulse’s wake would trigger lightning strikes, which would then travel along the plasma channels down to the ground, like a train running along railway tracks (Optics Express, DOI: 10.1364/OE.16.005757).

“We didn’t detect triggered lightning but we did detect electric activity synchronised with the laser pulses,” says Kasparian. He thinks the plasma was too low in energy to trigger full lightning strikes, but the researchers think they can solve that problem with modifications to the system.

Kasparian says that in future, Christodoulides’s team’s work could be combined with his to help aim the laser pulses and plasma channels at specific targets, such as clouds, although he points out that the laser pulses can also be guided using mirrors. “But it would be fun to see curved lightning discharges,” he says.

Journal reference: Science (DOI: 10.1126/science.1169544)