Abstract
We present a bright and coherent soft x-ray source based on high harmonic generation delivering up to 1010 photons per second centered at 120 eV within an 80 eV bandwidth. The source profits from fully phase-matched harmonic generation in an unmodulated hollow waveguide. Under these conditions, the resulting high harmonic spectrum is shown to be flat-top up to the cutoff photon energy and in line with the theoretical single-atom response. The source is characterized in view of seeding a free-electron laser and is shown to overcome the free-electron laser noise floor for wavelengths as short as 8.9 nm. This opens the perspective toward direct high harmonic seeding of a free-electron laser at soft x-ray wavelengths.
Highlights
One possible approach is quasi-phase matching where harmonic generation is suppressed in the region where harmonics interfere destructively [10]
We present significantly higher photon flux per shot up to 160 eV by using a conventional Ti:sapphire laser (0.8 μm) at a repetition rate compatible with FELs and at narrower bandwidth
In view of FEL seeding, we explore in this paper fully phase-matched HH generation in an unmodulated capillary hollow waveguide driven by a technologically mature Ti:sapphire laser system
Summary
In our experiment, performed at the Laboratoire d’Optique Appliquee, the 1 kHz Ti:sapphire laser delivers 40 fs full-width at half-maximum, 6 mJ pulses centered at 805 nm. The laser beam is focused with a lens ( f = 1.5 m) into a capillary that is continuously flooded with gas. Behind the capillary a set of metallic filters is used to separate the HH from the infrared radiation. A pair of flat and concave mirrors coated with ZrO2 are used to filter out the remaining infrared radiation and to focus the harmonic beam on the CCD. The spectrometer (dichroic mirror, focusing mirror and grating) could be removed to record the HH footprint. The spectrometer has been calibrated in wavelength using the absorption edge of aluminum and the zero order of the grating; see figure 1(b). The measurements are corrected with the transmission curves of the filters [16] and the quantum efficiency (QE) of the CCD camera. The reflectivity of the ZrO2 mirror for an incident angle of 10◦ is plotted, which indicates that the HH spectra are significantly affected by the drop in mirror reflectivity for photon energies in the cutoff region
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