LOA, Palaiseau
GoLP, Lisbon
PSI, Villigen
Free-electron lasers have been recently evolving very fast in the extreme-ultraviolet to soft X-ray region. Once seeded with high harmonics, these schemes are considered as next generation soft X-ray light sources delivering ultrashort pulses with high temporal and spatial coherence. Here we present a detailed experimental study of a kHz two-colour (fundamental + second harmonic) high harmonic generation and investigate its potential as a suitable evolution of the actual seeding sources. It turns out that this source (both odd and even harmonics) is highly tuneable, and delivers intense radiations with only one order of magnitude difference in the photon yield from 65 nm to 13 nm. We also observed an astonishing aberration-free character of these harmonics (aberration below λ/17 rms at 44 nm). Finally, the variable linear polarization of the harmonics was revealed to be easily controllable with the generation conditions. Then, the implementation of this technique on seeded FELs would allow amplifications, with perfect beam quality, to be achieved at wavelengths shorter than previously accessible.
LLNL, Livermore, California
Uppsala University, Biomedical Centre, Uppsala
DESY, Hamburg
Czech Republic Academy of Sciences, Institute of Physics, Prague
Queen's University of Belfast, Belfast, Northern Ireland
GoLP, Lisbon
IP PAS, Warsaw
FOM Rijnhuizen, Nieuwegein
SLAC, Menlo Park, California
STFC/RAL, Chilton, Didcot, Oxon
IMR SAS, Kosice
European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg
University of Oxford, Clarendon Laboratory, Oxford
We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: 13.5 nm, 15fs, 10μJ, 5Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si-ML giving a reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor control six-axis gimbel. Beam imprints on PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of <1μm producing intensities greater than 1016 Wcm−2. Observations were correlated with simulations of best focus to provide further relevant information. This focused XUV laser beam now allows us to begin exploring matter under extreme conditions. Future experimental efforts at ’4th generation’ light sources will be outlined.
University of Oxford, Clarendon Laboratory, Oxford
DESY, Hamburg
Rostock University, Faculty of Computer Science and Electrical Engineering, Rostock
Czech Republic Academy of Sciences, Institute of Physics, Prague
LLNL, Livermore, California
Queen's University of Belfast, Belfast, Northern Ireland
GoLP, Lisbon
IOQ, Jena
Rostock University, Rostock
UPMC, Paris
LBNL, Berkeley, California
IP PAS, Warsaw
Kohat University of Science and Technology, Kohat
FOM Rijnhuizen, Nieuwegein
SLAC, Menlo Park, California
UCB, Berkeley, California
SOLEIL, Gif-sur-Yvette
European X-ray Free Electron Laser Project Team, c/o DESY, Hamburg
FSU Jena, Jena
We report for the first time saturable absorption in the soft x-ray regime: by photoionizing L-shell core electrons we observed on a 15fs timescale a multifold increase of transmission through an aluminium foil. While saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths, it has never been observed in a core electron transition due to the short lifetimes of the created excited states and the high intensities of the soft x-rays that are needed. The experiments were performed at the XUV Free Electron Laser FLASH and used record high intensities. After the FEL pulse has passed, the aluminum sample is in an exotic state where all the aluminum atoms have a L-shell hole, and the conduction band has a 9eV temperature, while the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to Warm Dense Matter condition, at 20eV temperatures. The saturable absorption allows for a very homogeneous and efficient heating. Therefore the method is an ideal candidate to study homogeneous Warm Dense Matter, highly relevant to planetary science, astrophysics and inertial confinement fusion.