Recent advances in high-harmonic generation gave rise to soft X-ray pulses with higher intensity, shorter duration and higher photon energy. One of the remaining shortages of this source is its restriction to linear polarization, since the yield of generation of elliptically polarized high harmonics has been low so far. We here show how this limitation is overcome by using a cross-polarized two-colour laser field. With this simple technique, we reach high degrees of ellipticity (up to 75%) with efficiencies similar to classically generated linearly polarized harmonics. To demonstrate these features and to prove the capacity of our source for applications, we measure the X-ray magnetic circular dichroism (XMCD) effect of nickel at the M 2,3 absorption edge around 67 eV. There results open up the way towards femtosecond time-resolved experiments using high harmonics exploiting the powerful element-sensitive XMCD effect and resolving the ultrafast magnetization dynamics of individual components in complex materials.

No abstract

International audienceRecently several techniques demonstrated the production of elliptically polarized high harmonics. One of these techniques consists of the interaction in a noble gas of two-color laser beams having orthogonal linear polarizations. Here we present the theoretical explanation of such a result observed in Lambert et al. [Nat. Commun. 6, 6167 (2015)]. Numerical calculations based on the nonperturbative light-atom interaction theory reproduce well the experimental data. The degree of polarization is analyzed for different harmonic orders and found to be high. With the help of a simplified theoretical model it is shown that the degree of harmonic ellipticity depends mainly on the population of atomic state sublevels with different angular momentum projections