Imager sees femtosecond-scale birefringence

The ultrafast laser is a scientific tool unlike any other.

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The ultrafast laser is a scientific tool unlike any other. When focused, terawatt-level pulses produce large intrinsic electric and magnetic fields that accelerate ions. Also, the femtosecond pulse durations can provide snapshots of chemical reactions. Although technological uses of the ultrafast laser are already being discovered (see also supplement following p. 82 in this issue), they will take decades to explore—just as with many of the greatest inventions. Part of understanding just what femtosecond pulses can do comes from developing the right techniques for measuring their effects.

Researchers at Hamamatsu Photonics (Hamakita City, Japan) have demonstrated a device that produces a femtosecond-resolution series of images—akin to a movie—showing areas of instantaneous birefringence induced by a focused ultrafast laser pulse in air. When the individual frames are summed, the result is a spatial representation of a light cone that reveals filaments of high birefringence in a distribution very unlike what might be expected near a focus (see figure here and on cover). Such information is important not only to high-field physics in laser plasma interactions but also to inertial-confinement fusion and will undoubtedly be important in the future to ultrafast-laser-based industrial processing.

In the technique, called femtosecond time-resolved optical polarigraphy, light from a Ti:sapphire amplifier system is split into a pump beam and a probe beam. The amplifier emits pulses of 800-nm wavelength, 100-fs duration, 7-mJ energy, and 10-Hz repetition rate. The vertically polarized pump beam, which has a 1/e diameter of 12.8 mm and contains a variable optical delay line in its path, is focused to a point by a plano-convex lens of 50-mm focal length. The collimated 45µ-polarized probe beam intersects the focused pump beam at a right angle and then passes through an analyzer having a polarization perpendicular to that of the probe beam; thus, only light affected by birefringence passes through the analyzer. Relay lenses image the measurement region onto a charge-coupled-device (CCD) camera at a 4.8-µm/pixel spatial resolution. Any background light, such as the emission from the breakdown plasma, is subtracted out.

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PHOTO. Sum of 91 time-resolved images incremented by 66.7 fs shows birefringent behavior of air due to nonlinear effects near the focus of an ultrafast Ti:sapphire laser beam. The pump pulse is propagating from top to bottom. Dashed lines intersecting at the geometric focus indicate the 1/e intensity of the beam in vacuum (neglecting beam-waist effects).

In a typical measurement, 91 consecutive time-resolved images are taken, separated by 66.7-fs incremental steps introduced by the delay line. Because the profiles are stable on a shot-to-shot basis, 10 profiles are integrated to produce an averaged image. When the summing of images is done to obtain the spatial map of birefringence, the peak intensity of each image is normalized to show the extended shapes of the filaments, which weaken in intensity away from focus.

The researchers hypothesize that the filamentation originates from the absorption of the pump pulse energy or from ionization-induced refraction by the breakdown plasma, or both. The asymmetry in the filament structure may result from an asymmetry of the beam induced by its amplification or from a slight misalignment of the optics—although complex patterns appear even when the incident laser pulse has a Gaussian-like profile, according to Yutaka Tsuchiya, one of the researchers. Similar effects were seen in nitrogen, oxygen, carbon dioxide, the noble gases, and liquid carbon disulfide (CS2).

When the CCD is replaced with a photodiode and the pump-pulse energy is varied, the photodiode signal changes in proportion to the sinusoidal square of the energy, consistent with the optical Kerr effect. For larger energies, the response begins to level off due to absorption or ionization. The birefringence effect vanishes within 100 fs of the pulse departure for air.

Other interesting phenomena have made their appearance, says Tsuchiya. A single, long, stable light filament has been generated extending away from the focal point; Tsuchiya conjectures a self-channeling effect. With CS2 as the medium, many filaments appear and vanish at random in both space and time. "Sometimes the filaments have been attractive and combined with each other, have been repulsive, or in rare cases, have twisted like a spiral," says Tsuchiya.

John Wallace

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