Measurement of attosecond vortices
Using HHG, we managed to generate XUV beams with each harmonic carrying an OAM $\ell q$, where q is the harmonic order and $\ell$ the OAM of the driving laser. We observed in the far field a series of rings, all showing the same diameter for a given $\ell$, but scaling as the square root of $\ell$ for a given q. These observations could be reproduced with an analytical model, and by a full calculation from Thierry Auguste. We thereby demonstrated that the conservation rule of OAM was signed by this constant ring diameter throughout an harmonic spectrum.
We further managed to measure the spectral phase of this harmonic spectrum. With this data in hand, we could reconstruct the spatio-temporal structure of the generated attosecond pulse. It shows as a double helix, as predicted by Hernandez-Garcia et al.
This research was performed on LUCA laser in Saclay, in a collaboration between LCPMR in Paris (J\érémie Caillat and Richard Taieb) and the team of Lou Di Mauro in Ohio (Antoine Camper).
See also the press release Géneaux et al.
Generation of highly elliptically polarized harmonics
Chiral molecules, which are molecules that can not be superimposed to their mirror image (like the hands for instance), are ubiquitous in nature. Chirality is even sometimes proposed as a condition for the emergence of life. On more practical grounds, the two forms of a chiral chemical usually interact very differently with living organisms. The study of the dynamics of chiral molecules thus carries both fundamental and applied interests.
However, their dynamics at the femtosecond time scale and below is very poorly known, due to a lack of adapted diagnosis tools. In this publication, we show how a tuned HHG source can provide the perfect probe for these molecules, with at least a femtosecond time resolution. More precisely, tuning the polarization state of the driving laser, we managed to generate quasi circularly polarized harmonics. They were used to measure PhotoElectron Circular Dichroism (PECD) in a Velocity Map Imaging Spectrometer. This phenomenon, which is limited to chiral objects, has been largely studied on synchrotron quasi continuous sources ( Desirs com.). Measuring PECD with a femtosecond source now opens the way to time resolved studies.
This research was performed in Bordeaux, in a collaboration between CELIA-Bordeaux, the team of the DESIRS beamline on the SOLEIL synchrotron and the LCAR laboratory in Toulouse.
See also the press release on the CNRS website Ferré et al.