The HR GHHG Condensed beamline of ELI-ALPS is driven by the 100 kHz high average-power HR-1 laser system and produces broadband attosecond pulse trains (APTs) or monochromatic femtosecond pulses for XUV – IR pump-probe measurements in/on condensed-phase targets. It incorporates a time-delay compensated (TDC) monochromator to provide users with the possibility of selecting different XUV photon energy regions of the generated high-order harmonic radiation down to the 50 meV spectral bandwidth, while preserving the few (tens of) femtosecond duration of the XUV pulses. Transmission of the total available XUV spectrum (in “broadband” operation) or monochromatic pulses with longer temporal extent, but higher flux (in “high-flux” mode, HFM) is also possible. Different XUV spectral ranges and fluxes can be provided utilizing different rare gases for generation.
The beamline is under continuous upgrade and optimization, currently providing XUV radiation with the parameters listed in Table 1. Table 1 also shows the measured specifications of the HR-1 laser system used for generation. After the planned upgrade of the HR-1 laser system in the second half of 2021 and the arrival of the HR-2 laser in the second half of 2022, the specifications of the beamline are expected to substantially improve for experiments planned to be performed during 2022.
The schematic optical layout is given in Figure 1. The beamline is equipped with various diagnostic equipment including an electron time-of-flight (TOF) spectrometer, which serves as the primary tool for the temporal characterization of the XUV pulses via XUV-IR cross-correlation like measurements, positioned in “Target area 1”. In addition, a scintillator crystal in “Target area 1”, a flat-field XUV spectrometer and a photodiode can be used to monitor the spatial and spectral behaviour of the produced radiation together with the available photon flux, respectively. “Target area 2” currently has an angle-resolved energy-filtering photoemission microscope (NanoESCA) (see details at https://www.eli-alps.hu/en/Users-1/NanoESCA).
Figure 1. Schematic optical layout of the HR GHHG Condensed beamline.
Table 1. Measured parameters of the HR-1 laser system (in long pulse mode) and the XUV radiation generated through HHG.
Representative data of the selectable XUV bandwidth, and XUV-IR spectrograms for the temporal characterization of the monochromatic radiation are given in Figure 2 and Figure 3, respectively.
Figure 2. FWHM bandwidth of the selected high harmonic radiation in the low (a) and high (b) energy resolution modes (circle – ray-tracing simulation, star – measurement).
Figure 3. Experimental pump-probe traces recorded in low (a, b) and high-resolution (c, d) modes with the selection of the 29th harmonic with (a, c) and without (b, d) time-compensation using ~40 fs long IR pulse.