Task 2: Development of a high modulation rate elliptically polarized XUV source for XNCD and PECD measurements

To synthetize elliptically polarized XUV light through HHG, two approaches may be considered: either one tries to induce a dissymmetry in the generation process itself, or to post tailor the regular linearly polarized HHG radiation emitted from a full centro-symmetric generation scheme. Several ways have been proposed to disymmetrize the medium, playing either on the generating field or the generating medium itself. For the latter strategy, it has been shown lately that aligning an ensemble of linear molecules, one may observe ellipticities up to 50% [Zhou09]. This setup is however fairly cumbersome to use: it requires a cold target gas to be preliminary pumped in order to get an aligned ensemble of molecules. The price to pay is the density of emitters, hence the total photon flux. This approach which was also implemented in our laboratory [Diveki11] , will be kept as a backup solution. Alternatively, it appears less demanding to tailor the incoming field. Two ways are currently demonstrated: either performing HHG with an incoming elliptically polarized field [Antoine97] , or with a two color (e.g. 400nm + 800 nm) circularly polarized field [Eichmann95] . For our demonstration of PECD measurements (figure), it is the former approach that we used. Here again a tradeoff must be done between the efficiency of HHG in an elliptically polarized field and the degree of ellipticity obtained.

Synthesis of XUV femtosecond beam carrying a SAM carrying out HHG with an elliptically polarized beam in SF6. This approach was optimized in the CELIA Lab in Bordeaux, together with Yann Mairesse Valérie Blanchet and coworkers [Ferre2014]

Finally, a straightforward approach is to generate linearly polarized harmonics and shape them after the HHG stage using a multi-mirror XUV circular polarizer [Vodungbo11]. Although the transmission of the circular polarizer is only a few percent, since the harmonics are generated with linearly polarized light, the efficiency may be comparable to the generation with tailored incoming beams. This method is however way more costly than the others.

Once optimized, the elliptically polarized XUV source will be used for demonstrations of PECD measurement and XNCD, first in the static regime, second in pump probe schemes.