FEATURE ARTICLE
Filaments of Light
Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light
Jérôme Kasparian
Next time you give a presentation about your research, take a close
look at the laser pointer you're holding in your hand. How big is
the beam coming out of it? And how large is the spot that it forms?
The answers will, of course, hinge on the particular laser pointer
you're wielding and the distance between podium and screen. Typical
values might be a few millimeters for the beam as it exits the
aperture of the pointer and a centimeter or so for the circle of
light it casts across the auditorium. It takes only a smattering of
physical intuition to guess the reason: Diffraction causes the beam
to diverge. The actual cause may be a little more complicated,
because some laser pointers include a lens that makes the light
converge at a fixed distance from the tip, which leads the beam to
spread out beyond this focal point—more so than if only
diffraction had operated.
Imagine now that your laser pen packed a more powerful
punch—say that the intensity of the beam was a whopping
1012 times that of a typical pointer. What then would the
beam do as it crossed the room? (It's clear enough what it'll do
when it hits the screen—quickly burn a hole). The answer, it
turns out, is anything but intuitive. A laser of sufficient
intensity traveling through air will—all by
itself—engineer a narrow channel, one perhaps a tenth of a
millimeter wide, over which light will propagate for tens or even
hundreds of meters. Such filaments of laser light were first created
a little more than a decade ago, and investigators are just now
beginning to explore a variety of applications for
them—mapping atmospheric pollutants, characterizing materials
at a distance, perhaps one day even controlling lightning.
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