2021, Major, Balázs, Ghafur, Omair, Kovács, Katalin, Varjú, Katalin, Tosa, Valer, Vrakking, Marc J. J., Schütte, B.

High-intensity laser pulses covering the ultraviolet to terahertz spectral regions are nowadays routinely generated in a large number of laboratories. In contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated using a small number of sources including free-electron laser facilities [1-3] and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a concept for a compact intense XUV source based on HHG that is focused to an intensity of $2 \times 10^{14}$ W/cm$^2$, with a potential increase up to $10^{17}$ W/cm$^2$ in the future. Our approach uses tight focusing of the near-infrared (NIR) driving laser and minimizes the XUV virtual source size by generating harmonics several Rayleigh lengths away from the NIR focus. Accordingly, the XUV pulses can be refocused to a small beam waist radius of 600 nm, enabling the absorption of up to four XUV photons by a single Ar atom in a setup that fits on a modest (2 m) laser table. Our concept represents a straightforward approach for the generation of intense XUV pulses in many laboratories, providing novel opportunities for XUV strong-field and nonlinear optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent diffractive imaging of nanoscale structures.

2015, Tanyag, Rico Mayro P., Bernando, Charles, Jones, Curtis F., Bacellar, Camila, Ferguson, Ken R., Anielski, Denis, Boll, Rebecca, Carron, Sebastian, Cryan, James P., Englert, Lars, Epp, Sascha W., Erk, Benjamin, Foucar, Lutz, Gomez, Luis F., Hartmann, Robert, Neumark, Daniel M., Rolles, Daniel, Rudek, Benedikt, Rudenko, Artem, Siefermann, Katrin R., Ullrich, Joachim, Weise, Fabian, Bostedt, Christoph, Gessner, Oliver, Vilesov, Andrey F.

Lensless x-ray microscopy requires the recovery of the phase of the radiation scattered from a specimen. Here, we demonstrate a de novo phase retrieval technique by encapsulating an object in a superfluid helium nanodroplet, which provides both a physical support and an approximate scattering phase for the iterative image reconstruction. The technique is robust, fast-converging, and yields the complex density of the immersed object. Images of xenon clusters embedded in superfluid helium droplets reveal transient configurations of quantum vortices in this fragile system.