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Lasers & Sources

How low can you go?

Eye on Technology - ultrafast lasers

From oemagazine February 2002
31 February 2002, SPIE Newsroom. DOI: 10.1117/2.5200202.0001

There's an old saying in optical fabrication: If you can't measure it, you can't make it. In work that has made the ultrafast community sit up and take notice, researchers have not only produced attosecond (as) x-ray pulses, they've also developed a method to reliably measure them. Ferenc Krausz and collaborators from the Vienna University of Technology (TU Wien; Vienna, Austria), the Steacie Institute for Molecular Sciences (SIMS; Ottawa, Canada), and the University of Bielefeld (Bielefeld, Germany) used a high-order harmonic generation by a few-cycle driver pulse to produce single 90 eV soft-x-ray pulses of 650 ± 150 as in duration; then they measured the pulses using a visible/soft-x-ray cross-correlation scheme.

Using a laser system supplying 7 fs driver pulses of 700 µJ energy at 750 nm and 1 kHz repetition rate, the group focused ~1015 W/cm2 of optical power onto a 3-mm-long, 200-mbar neon target, generating an attosecond pulse, collinear beam; filtering removed all but the highest order (n > 55) harmonics.

The group directed the two beams to the krypton target by a monolithic laser/soft-x-ray cross correlator consisting of two concentric mirrors--an inner 3-mm diameter part with molybdenum/silicon multilayer coating and an outer 10-mm diameter part for the laser radiation. By recording the modulation of the width of the freed electron kinetic energy distribution, the group deduced the attosecond x-ray pulse duration.

The contour plot shows the modulation of the kinetic energy distribution of photoelectrons arising from a gas of krypton atoms exposed simultaneously to the two collinear beams. As the x-ray pulse is scanned through the laser pulse, the photoelectron spectrum broadens and contracts once over half the laser field oscillation cycle (~ 1.25 fs), displaying thereby the oscillating laser electric field. At the center of the laser pulse a sudden decrease in the oscillation period due to an abrupt blue shift of the laser carrier frequency is observed.

To measure the pulse duration, the group used the x-ray pulse to photoionize atomic krypton gas in the presence of the driver pulse. The x-ray photons trigger the ejection of krypton electrons with varying angular distributions of momenta modulated by the oscillating laser field. Momentum transferred parallel to the polarization of the driver pulse downshifts the kinetic energy of the electrons traveling orthogonal to the polarization and broadens their spectrum. The spectral broadening is subject to modulation as the x-ray pulse is scanned through the laser pulse. Because the laser pulse carried a strong and sudden frequency shift at its peak, from this cross correlation the group could not only determine the duration of the sub-fs x-ray pulse but also rule out the existence of substantial satellite pulses preceding or following the main x-ray pulse. Cross-correlation techniques allowed the group to evaluate the variation in kinetic energy between the absorption of the x-ray pulse and the peak of the driver pulse, thus obtaining the pulse duration.

"Because the generated sub-fs x-rays are too weak to be used as a pump and as a probe pulse simultaneously, our 'two-color' illumination scheme offers the only feasible route to attosecond time-resolved spectroscopy at present," explains Krausz. Future work will aim to optimize the setup to further compress the pulse duration of the x-ray pulses. The team's generation and characterization method offers sub-fs resolution, which effectively paves the way for experimental attosecond science.