A. L'Huillier, J. Mauritsson, P. Johnsson. T. Remetter, E. Gustafsson, T.
Ruchon and M. Swoboda,
Department of Physics, Lund University, P. O. Box 118, SE-221 00
The characteristic plateau region of the harmonic spectrum produced when an atom is ionized in an intense infrared laser field spans from the ultraviolet into the soft X-ray region, thus providing enough bandwidth to produce pulses of hundred attoseconds. However, the different frequency components in the harmonic spectrum are not naturally synchronized. Ensuring or imposing a sufficient degree of synchronization over a certain spectral bandwidth, combined with the filtering of this bandwidth is the biggest problem that must be overcome for the production of short attosecond pulses. In this communication, we report on the different technologies that can be used to spectrally filter and phase control high-order harmonics in order to produce sub-200 as pulses on target in a wide energy region [1-4].
Applications of attosecond pulses are emerging. We will present applications of attosecond pulses to time-resolved studies of ionization of rare gas atoms by attosecond pulses in presence of an infrared laser pulse [3-5] and to interferometric measurements of electron wave packets . Our experiments consist in measuring the angular and energy-resolved electron emission from atoms exposed to a train of attosecond pulses in presence of an infrared laser field using a velocity map imaging technique. The momentum distributions depend on the timing of injection of the electron wave packets in the continuum relative to the laser cycle. We will especially discuss the results presented in  where the interferences between wave packets created at a given time in the infrared cycle and those created half a cycle later are studied as a function of the relative delay between the infrared field and the attosecond pulses. In some cases, information on the phase of the electron wave packet can be extracted from the interferograms, in a way resembling spectral-shearing interferometry of optical pulses.
 R. López-Martens et al, Phys. Rev. Lett. 94, 033001 (2005)
 A.-S. Morlens et al., Opt. Lett. 31, 1558 (2006)
 P. Johnsson et al., in preparation
 J. Mauritsson et al., Phys. Rev. Lett., in press (2006)
 P. Johnsson et al., Phys. Rev. Lett. 95, 013001 (2005)
 T. Remetter et al, Nature Physics 2, 323 (2006)
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