2021, Hort, O., Dubrouil, A., Khokhlova, M.A., Descamps, D., Petit, S., Burgy, F., Mével, E., Constant, E., Strelkov, V.V.

Extreme ultraviolet (XUV) radiation finds numerous applications in spectroscopy. When the XUV light is generated via high-order harmonic generation (HHG), it may be produced in the form of attosecond pulses, allowing access to unprecedented ultrafast phenomena. However, the HHG efficiency remains limited. Here we present an observation of a new regime of coherent XUV emission which has a potential to provide higher XUV intensity, vital for applications. We explain the process by high-order parametric generation, involving the combined emission of THz and XUV photons, where the phase matching is very robust against ionization. This introduces a way to use higher-energy driving pulses, thus generating more XUV photons.

2015, Ferré, A., Boguslavskiy, A.E., Dagan, M., Blanchet, V., Bruner, B.D., Burgy, F., Camper, A., Descamps, D., Fabre, B., Fedorov, N., Gaudin, J., Geoffroy, G., Mikosch, J., Patchkovskii, S., Petit, S., Ruchon, T., Soifer, H., Staedter, D., Wilkinson, I., Stolow, A., Dudovich, N., Mairesse, Y.

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20-26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.