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The introduction of novel experimental methods for the generation and control of extreme ultraviolet (XUV) spectra via high order harmonic generation (HHG) has determined the rapid progress in the field of attosecond science. A new experimental technique for the generation of XUV continua has been recently demonstrated that is based on the polarization gating method with few-cycle phase-stabilized driving pulses. Using aluminum filters of different thicknesses, the chirp of the attosecond pulses can be compensated (e.g. with argon and a 300 nm thick aluminum filter nearly single-cycle 130-as pulses were created) [1]. The carrier-envelope phase of the IR laser and the width of the polarization gate can be adjusted to vary the number of attosecond pulses that are created upon subsequent half-cycles of the IR driving field. We have utilized velocity-map imaging (VMI) in combination with this XUV source to record the dynamics of attosecond electron wave packets in the presence of a strong IR-field [2]. Electron wave packets were created upon XUV-ionization of helium. Using an isolated attosecond pulse, the momentum shift of the electron wave packet in the IR-field can be observed with full angular resolution. An overall periodic energy shift of the electron wave packet with a period equal to the IR oscillation period is seen, thus confirming the generation of isolated attosecond pulses. We point out that the oscillations of the photoelectron energy in the up- and downwards directions are 180° out of phase. A striking feature of the measurements is the presence of clear interference patterns in the energy distribution. The appearance of these fringes depends critically on the delay between the XUV and IR pulses. Also the formation of the fringes appears to be out-of phase in the up and down directions. The interpretation of these results is in progress.
References [1] G. Sansone et al., "Isolated Single-Cycle Attosecond Pulses", Science 314, 443 (2006). [2] T. Remetter et al., "Attosecond Electron Wave Racket Interferometry", Nature Physics 2, 323 (2006). |
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