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.
Highlights
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
Ultrashort table-top high-harmonic generation (HHG) sources have long been confined to low ellipticities[10,11,12,13,14,15] until recently when various schemes were shown to produce a high degree of circularity[4,16,17,18,19,20,21]
Significant deviations from circularity and depolarization are evidenced by a complete experimental characterization of the polarization state of high harmonics generated in argon, performed with the molecular polarimetry method[41,42]
Summary
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 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. The recent theoretical and experimental effort has concentrated on the control of the intensity ratio between the 3q+1 and the 3q+2 orders[30], using phase matching[18,36], the electronic structure of the generation medium[28,31,37,38], the intensity of the driving fields[39], or, in a very early study, a strong magnetic field[40] This ratio is important for determining the polarization of the attosecond pulse train (APT) composed of all harmonic orders. The molecular frame photoelectron angular distributions (MFPAD) characterizing photoionization of NO molecules provide simultaneously all three Stokes parameters of the XUV light—including the challenging disentanglement of the circular and unpolarized components
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