2020, Major, B., Kretschmar, M., Ghafur, O., Hoffmann, A., Kovács, K., Varjú, K., Senfftleben, B., Tümmler, J., Will, I., Nagy, T., Rupp, D., Vrakking, M.J.J., Tosa, V., Schütte, B.

The ongoing development of intense high-harmonic generation (HHG) sources has recently enabled highly non-linear ionization of atoms by the absorption of at least 10 extreme-ultraviolet (XUV) photons within a single atom (Senfftleben et al, arXiv:1911.01375). Here we investigate how the generation of these very intense HHG pulses in our 18-m-long beamline is aided by the reshaping of the fundamental, few-cycle, near-infrared (NIR) driving laser within a 30-cm-long HHG Xe medium. Using an incident NIR intensity that is higher than what is required for phase-matched HHG, signatures of reshaping are found by measuring the NIR blueshift and the fluorescence from the HHG medium along the propagation axis. These results are well reproduced by numerical calculations that show temporal compression of the NIR pulses in the HHG medium. The simulations predict that after refocusing an XUV beam waist radius of 320 nm and a clean attosecond pulse train can be obtained in the focal plane, with an estimated XUV peak intensity of 9 × 1015 W cm-2. Our results show that XUV intensities that were previously only available at large-scale facilities can now be obtained using moderately powerful table-top light sources. © 2020 The Author(s). Published by IOP Publishing Ltd

2010, Mauritsson, J., Remetter, T., Swoboda, M., Klünder, K., L'Huillier, A., Schafer, K.J., Ghafur, O., Kelkensberg, F., Siu, W., Johnsson, P., Vrakking, M.J.J., Znakovskaya, I., Uphues, T., Zherebtsov, S., Kling, M.F., Lépine, F., Benedetti, E., Ferrari, F., Sansone, G., Nisoli, M.

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration. © 2010 The American Physical Society.