The adage 鈥淓veryone complains about the weather but nobody does anything about it,鈥� may one day be obsolete if researchers at the 麻豆原创鈥檚 College of Optics & Photonics and the University of Arizona further develop a new technique to aim a high-energy laser beam into clouds to make it rain or trigger lightning.
The solution? Surround the beam with a second beam to act as an energy reservoir, sustaining the central beam to greater distances than previously possible. The secondary 鈥渄ress鈥� beam refuels and helps prevent the dissipation of the high-intensity primary beam, which on its own would break down quickly. A report on the project, 鈥淓xternally refueled optical filaments,鈥� was recently published in Nature Photonics.
Water condensation and lightning activity in clouds are linked to large amounts of static charged particles. Stimulating those particles with the right kind of laser holds the key to possibly one day summoning a shower when and where it is needed.
Lasers can already travel great distances but 鈥渨hen a laser beam becomes intense enough, it behaves differently than usual 鈥� it collapses inward on itself,鈥� said Matthew Mills, a graduate student in the Center for Research and Education in Optics and Lasers (CREOL). 鈥淭he collapse becomes so intense that electrons in the air鈥檚 oxygen and nitrogen are ripped off creating plasma 鈥� basically a soup of electrons.鈥�
At that point, the plasma immediately tries to spread the beam back out, causing a struggle between the spreading and collapsing of an ultra-short laser pulse. This struggle is called filamentation, and creates a filament or 鈥渓ight string鈥� that only propagates for a while until the properties of air make the beam disperse.
鈥淏ecause a filament creates excited electrons in its wake as it moves, it artificially聽seeds聽the conditions necessary for rain聽and lightning to occur,鈥� Mills said. Other researchers have caused 鈥渆lectrical events鈥� in clouds, but not lightning strikes.
But how do you get close enough to direct the beam into the cloud without being blasted to smithereens by lightning?
鈥淲hat would be nice is to have a sneaky way which allows us to produce an arbitrary long 鈥榝ilament extension cable.鈥� It turns out that if you wrap a large, low intensity, doughnut-like 鈥榙ress鈥� beam around the filament and slowly move it inward, you can provide this arbitrary extension,鈥� Mills said. 鈥淪ince we have control over the length of a filament with our method, one could聽seed the conditions needed for a rainstorm from afar. Ultimately, you could artificially聽control the rain and lightning over a large expanse with such ideas.鈥�
So far, Mills and fellow graduate student Ali Miri have been able to extend the pulse from 10 inches to about 7 feet. And they鈥檙e working to extend the filament even farther.
鈥淭his work could ultimately lead to ultra-long optically induced filaments or plasma channels that are otherwise impossible to establish under normal conditions,鈥� said professor Demetrios Christodoulides, who is working with the graduate students on the project.
鈥淚n principle such dressed filaments could propagate for more than 50 meters or so, thus enabling a number of applications. This family of optical filaments may one day be used to selectively guide microwave signals along very long plasma channels, perhaps for hundreds of meters.鈥�
Other possible uses of this technique could be used in long-distance sensors and spectrometers to identify chemical makeup. Development of the technology was supported by a $7.5 million grant from the Department of Defense.