The XUV is produced through high harmonic generation (HHG) in a gas target. The laser is focused to an intensity of ~1014
in a differentially pumped gas cell and up to one part in 106
of the energy is converted to short pulses of XUV radiation in the 10 - 100 nm (10 - 100 eV) range. The XUV pulses have a similar pulse-duration to the drive laser pulse (~30 fs) and are synchronised to it with sub-fs resolution.
Tuneable XUV beamline
Artemis provides a tuneable XUV beamline for experiments where wavelength and bandwidth selection is required. In this beamline, the harmonics pass through a specially designed XUV monochromator, which enables a single harmonic to be selected from the spectrum, while maintaining the femtosecond pulse-duration.
The XUV monochromator was designed and built in collaboration with the LUXOR Laboratory of the Italian National Research Council. The monochromator has four interchangeable diffraction gratings to cover two spectral ranges (12-30 nm and 30-90 nm) with either higher resolving power or shorter pulse duration. The grating resolving power is limited to ~100 in order to keep the XUV pulse duration below 60 fs. The measured peak transmission of the monochromator is 30%. The XUV flux after the monochromator is typically 1.8x109
photons/second at 30 eV. Single, isolated harmonics can be selected at photon energies up to 80 eV. The monochromator is described in detail in
Optics Express 19 191690 (2011)
. The typical XUV pulse duration is ~30 fs.
The XUV monochromator can also be used with few-cycle pulses from the hollow fibre. We have made time-resolved ARPES measurements demonstrating 8 fs temporal resolution with 800 meV energy resolution [Gierz, Phys Rev Lett 115 086803 (2015)]
Using the XUV monochromator on Artemis to select a single harmonic has advantages over multilayer mirrors. First, the photon energy and resolution can be easily tuned over the HHG spectrum. Second, no metal filter is needed between the harmonic generation and the monochromator chamber, which improves the transmission through the beamline.
The XUV pulses are refocused into the end-stations with a gold-coated toroidal mirror used at grazing incidence. They can be recombined inside the end-station with laser pulses at wavelengths from the ultraviolet to the far-infrared for pump-probe experiments.