From Femtochemistry to Attophysics

Amid a fast game in a vast venue, sports photography seeks to freeze motion and isolate small portions of space for special consideration. In the scientific world of the ultrafast and ultrasmall, stroboscopic effects are achieved with greatly attenuated laser pulses. The advent of laser light served up in femtosecond (or 10^-15 second) bursts has helped to elucidate the molecular world by freezing their vibrational and rotational motions. Scientists would of course like to instigate and monitor even shorter times and distances.

A collaboration between scientists at the Technical University of Vienna and the Max Planck Institute for Quantum Optics (MPQ) has now done precisely this. They have produced a series of 5-fsec pulses, each consisting of only a few cycles of a carrier light signal modulated within an amplitude envelope. In the case of the Vienna-MPQ experiment, however, all the pulses are identical (a feat not achieved previously) and the phase of the carrier wave within the envelope is controlled with a time resolution of about 100 attoseconds.

When the intense (100 GW) few-cycle pulse strikes an atom, an electron can be stripped away quickly, and reabsorbed just as quickly. This violent excursion results in the emission of a sharp x-ray spike with a duration even shorter than the pulse that excited the reaction. In fact the x-ray pulses are about 500 attoseconds long. Moreover, because all the waveforms of the optical pulse are identical, and controlled, the subsequent electron motions and x-ray emissions are also highly controlled and reproducible. At a talk at this week's meeting of the American Association for the Advancement of Science (AAAS) in Denver, Vienna physicist Ferenc Krausz said that this sub-femtosecond control of electron currents represented true attophysics, a new technique for directing and watching atomic processes at unprecedentedly short time intervals. (See Baltuska et al., Nature, 6 February 2003.)