Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly polarized fields

Lou Barreau 1 Kévin Veyrinas 2 Vincent Gruson 1 Sébastien J. Weber 3 Thierry Auguste 1 Jean-François Hergott 4 Fabien Lepetit 4 Bertrand Carre 1 Jean-Christophe Houver 2 Danielle Dowek 2 Pascal Salières 1
1 ATTO - Attophysique
IRAMIS - Institut Rayonnement Matière de Saclay, LIDyl - Laboratoire Interactions, Dynamique et Lasers (ex SPAM)
4 SLIC - Serveurs Laser
IRAMIS - Institut Rayonnement Matière de Saclay, LIDyl - Laboratoire Interactions, Dynamique et Lasers (ex SPAM)
Abstract : High harmonics generated by counter-rotating laser fields at the fundamental and second harmonic frequencies have raised important interest as a table-top source of circularly polarized ultrashort extreme-ultraviolet light. However, this emission has not yet been fully characterized: in particular it was assumed to be fully polarized, leading to an uncertainty on the effective harmonic ellipticity. Here we show, through simulations, that ultrashort driving fields and ultrafast medium ionization lead to a breaking of the dynamical symmetry of the interaction, and consequently to deviations from perfectly circular and fully polarized harmonics, already at the single-atom level. We perform the complete experimental characterization of the polarization state of high harmonics generated along that scheme, giving direct access to the ellipticity absolute value and sign, as well as the degree of polarization of individual harmonic orders. This study allows defining optimal generation conditions of fully circularly polarized harmonics for advanced studies of ultrafast dichroisms.
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Lou Barreau, Kévin Veyrinas, Vincent Gruson, Sébastien J. Weber, Thierry Auguste, et al.. Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly polarized fields. Nature Communications, Nature Publishing Group, 2018, 9, pp.4727. ⟨10.1038/s41467-018-07151-8⟩. ⟨cea-01934789⟩

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