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Study Confirms Attosecond Pulses

Photonics Spectra
Jul 2000
Aaron J. Hand

Although researchers have achieved laser pulses as short as a few femtoseconds, attosecond pulses have proved prohibitively difficult -- until now. A group at the Foundation for Research and Technology-Hellas has produced and experimentally demonstrated trains of pulses lasting about 100 as.

Just as femtosecond lasers have enabled chemists to study fast chemical processes in real time, attosecond lasers could open up the observation of electronic motion. Electrons, which move much faster than atoms, underlie such dynamic phenomena as ionization, chemical bonding and delocalization of charges in large molecules, said Nektarios A. Papadogiannis, a researcher at the foundation's Institute of Electronic Structure and Laser.

The production of the extremely short pulses requires a completely different generation technique than that used for femtosecond pulses, Papadogiannis said. It needs a frequency bandwidth larger than 1 PHz, which requires extreme-UV radiation. "Such a bandwidth cannot be supplied by the gain curve of any of the existing lasing materials," he said. "That is why all proposed schemes for attosecond generation must rely on higher-order harmonic generation." Although the generation of attosecond pulses may be common in such experiments, measuring those pulses is a much more difficult matter, since standard methods destroy the pulses.

Pulses overlap

The Greek researchers took an unconventional approach, based in principle on high-resolution time-domain spectroscopy. Excitation and probing must occur within the laser pulse duration. So they partially overlap the pump and probe laser pulses in time, generating and probing the attosecond pulses simultaneously.

"Although this method is appropriate for the demonstration of attosecond localization, the entanglement of production and measurement makes the modeling of the experiment a rather demanding issue," Papadogiannis said.

Described in the Nov. 22, 1999, issue of Physical Review Letters, the experiments consist of three main steps: A fundamental 60-fs, 1-mJ pulse from a Ti:sapphire laser passes through a Michelson interferometer, creating two collinear femtosecond pulses with mutually tunable time delay with attosecond accuracy. Aimed into a noble gas, the two intense IR pulses generate odd higher-order harmonics. From those, the researchers select a set of harmonics with fixed-phase relation and measure their total intensity as a function of the delay time between the two laser pulses, thus measuring the attosecond beating.

The team uses a laser from BM Industries (now part of Thomson-CSF). Because attosecond pulses are so fragile, however, their use will require major improvements in extreme-UV optics, Papadogiannis said. "This will be one of the most difficult tasks in this research field."

The researchers plan to experiment further, manipulating crucial generation parameters to control the pulse characteristics, he said.

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