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A high-power optical waveguide in air

Sound and heat in the wake of a femtosecond laser pulse can be used to produce a long-lived refractive-index gradient that steers subsequent higher-power pulses.

If a laser pulse propagating through air is intense enough, it focuses itself—no lens required—thanks to the slight dependence of the index of refraction on light intensity. But when the pulse’s energy density grows sufficiently high, it ionizes nearby atoms and creates a plasma that defocuses the pulse until the intensity falls below the ionization threshold. That dynamic interplay between focusing and defocusing creates what’s known as a filament, a pulse that acts as its own waveguide. Such pulses can cover kilometers without diffracting (see Physics Today, August 2001, page 17). Applications abound but suffer from a persistent limitation: Single filaments cannot deliver more than about a watt of power. The University of Maryland’s Howard Milchberg and colleagues have now found a clever way to use filaments to guide subsequent laser pulses of much higher power. Using a titanium-sapphire laser, they shine a single femtosecond pulse through a pair of phase-shifting cellophane films to produce a square array of four filaments. The filaments act as impulsive heat sources: Four acoustic waves are launched from the corners and constructively interfere for about 100 ns in the center of the array, which increases the index of refraction n there, as shown in the figure. Later, four depressions in the gas density remain in the corners with higher density in the center that gradually dissipates by thermal diffusion. The result is a waveguide that persists for milliseconds, long enough to channel a stream of high-energy pulses—with average powers projected to be on the megawatt scale. (N. Jhajj et al., Phys. Rev. X, in press.)—R. Mark Wilson

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Scitation: A high-power optical waveguide in air