A. Giulietti, A. André, S. Dobosz Dufrénoy, D. Giulietti, T. Hosokai, P. Koester,
H. Kotaki, L. Labate, T. Levato, R. Nuter, N. C. Pathak, P. Monot, L. A. Gizzi
Phys. Plasmas 20, 082307 (2013).
A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90 degrees Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space-and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as delta lambda/delta t approximate to 3 angstrom/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a(0) << 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range.
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