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Filaments of Light

Pulsed terawatt lasers create some surprising effects when shone through the air—including the channeling of light

Jérôme Kasparian

Figure 8. In an experiment...Click to Enlarge Image

When shone through the atmosphere, a laser beam of sufficient intensity does not spread out as it propagates. Rather, it travels along a narrow filament, one that is perhaps 0.1 millimeter wide. This curious phenomenon arises because the refractive index of air depends on the intensity of the light, which is highest at the center of the beam, causing it to come to a focus—as if it was directed through a convex lens. This self-focusing effect continues only up to a point: Eventually the intensity of the focused beam becomes so high that electrons are stripped from the oxygen and nitrogen atoms in the air. Such ionization would be expected to cause the beam to diverge, but in combination with the self-focusing that is also taking place, it allows light to travel for tens or even hundreds of meters as a narrow filament. The author and his colleagues have constructed a mobile terawatt laser that can create such filaments of light, which can be used (among other things) to probe the nature of aerosol particles.

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