Wednesday, July 02, 2008

Light Pulse Speed Record Set

Researchers have set a new record in ultrafast metrology, producing the first light pulses lasting only 80 attoseconds (a billionth of a billionth of a second).

The 80-attosecond achievement marks the first time scientists have achieved light pulse speeds below 100 attoseconds and was accomplished by a team of physicists led by professor Ferenc Krausz at the Max Planck Institute for Quantum Optics (MPQ) in Garching and professor Ulf Kleineberg at Ludwig Maximilians University Munich, working in cooperation with colleagues at the Advanced Light Source at Berkeley Lab in California.

To generate attosecond pulses, the Garching physicists use the strong electric field of flashes in the near-infrared spectrum. In the hypershort laser flashes this field performs hardly more than a single strong oscillation with a period of about 2.5 femtoseconds (a femtosecond is 1000 attoseconds). That is: the light wave now comprises just two high wave peaks and a deep wave valley between them. The force exerted by the electric light field on the electrons is strongest at the summits and the lowest point of the valley; strong enough to liberate electrons which are ejected from rare-gas atoms in the experiment at Garching. This leaves ion rumps.

The vacuum chamber for attosecond metrology: Attosecond pulses of extreme ultraviolet light (depicted as a blue beam) are focused by a mirror (right) on a jet of neon atoms effusing from a thin valve. At the same time an infrared beam is striking the atoms. Both beams in combination allow real-time observation of the motion of electrons in the neon atoms and measurement of the duration of the attosecond pulse.

With the oscillation of the light field the force changes direction and very soon hurls the electrons back to the ion rumps. The recolliding free electrons induce extremely fast electron oscillations which last just attoseconds and emit light flashes of the same duration. These flashes are then in the region of extreme ultraviolet light (XUV, a wavelength of approximately 10 to 20 nm).

Controlled production of this single strong light oscillation within a hypershort flash has now allowed the Garching research team for the first time to release electrons exactly three times during a single laser pulse. On returning to the ion they then emit exactly three attosecond pulses. Each femtosecond laser flash generates three attosecond pulses. One of these pulses has a particularly high intensity, providing more than 100 million photons in a period of just 80 attoseconds.

This pulse is filtered out with special x-ray mirrors from Kleineberg, resulting in a single isolated x-ray pulse lasting 80 attoseconds.

A paper on their work, "Single-cycle Nonlinear Optics," appeared in the June 20 edition of Science.


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