VUV source for two-color dynamics studies at FLASH2

Abstract

Free electron lasers (FELs) and high order harmonic generation (HHG) sources produce ultrafast coherent extreme ultraviolet (XUV) and X-ray radiation. These two powerful sources are optimal tools for time-resolved studies of ultrafast dynamics due to their high spatial and temporal resolution. Common time-resolved studies make use of pump-probe schemes, where a first pulse triggers a dynamics and a second delayed pulse probes the status of the target at a known moment in time. Split-and-delay units are often employed in order to separate the pump and probe pulses provided by a single high photon flux source. Alternatively, a second synchronized source can be added for two-color experiments. In this case, the two photon energies are chosen accordingly to the process under investigation. Typically, the spectral range covered by secondary sources spans from the ultraviolet to the THz range. However, the parameter space can be extended by combining two separate short-wavelength systems, such as a FEL and a HHG-based beamline. Such con figuration bene fits from the sources complementary properties, opening a new regime for two-color studies in the XUV/X-ray range.

The permanent integration of a HHG-based source in a FEL endstation was recently realized for the first time at the free electron laser FLASH and is presented in this thesis work. Preliminary studies on the source confi guration and pressure calculations allowed to design a compact and reliable system, able to encounter the challenging vacuum requirements given by the ultra high-vacuum FEL endstation. The HHG process is driven by a OPCPA laser system, synchronized with the unique FEL burst timing, which delivers femtosecond pulses used for the generation of vacuum ultraviolet (VUV) light in a gas target. The emitted radiation is resolved in a compact in-line spectrometer and can be spectrally tuned by means of a rotatable double fi lterwheel. A hyperboloidal carbon-coated mirror allows for the coupling in the FEL endstation where the VUV beam is focused into a reaction microscope (REMI). First commissioning results have shown the generation of radiation from 10 to 40 eV with pulse energies up to 0.1 nJ and the successful focusing in REMI down to a 20 μm diameter spot. Photo-electron momentum measurements in argon indicate the source capabilities for future pump-probe experiments.