29 Nov 2018

First experiment on generation of high-order harmonic radiation

At the beginning of September, the laser team and the E1 experimental team reached an important milestone in ELI Beamlines’ technology integration. The first joint experiment on generation of high-order harmonics (HHG) in the HHG Beamline in E1 hall with L1-ALLEGRA laser driver was successfully performed.

This experiment was a culmination of a year-long installation of the L1-ALLEGRA laser, the L1-E1 beam transport, and of the HHG beamline, which were integrated into one system for user experiments. 

In this first integrated test experiment, the ALLEGRA laser front-end output was propagated through the whole system and compressed in vacuum by chirped mirrors resulting in the following laser output parameters: Pulse energy: 1.4 mJ, pulse duration: 14 fs, central wavelength: 830 nm, repetition rate: 1 kHz. The beam was then propagated though the E1 vacuum beam transport delivering it to the HHG beamline. There it was characterized and focused into a 2 cm long gas cell. Good pointing stability was measured, which is a crucial feature for pump-probe experiments with tight focusing.

The harmonic generation performance of L1 was compared to the previously available commercial laser driver (Coherent Astrella: 5 mJ, 40 fs, 1 kHz) used in E1. The L1 driven spectrum was found to have higher efficiency below 30 nm and generate shorter wavelengths (see Fig.2).

Fig. 2. Spectrum of the high order harmonics generated in gas cell filled with argon driven by a commercial laser driver with 40 fs-long pulses (blue) and with L1 laser pulses (red). The harmonics cutoff in logarithmic scale is shown in the inset.

The experimental team will continue research and development of the kHz X–ray sources in E1, namely the plasma X-ray source, which generates 10 keV radiation in sub-picosecond pulses.

Fig. 1 HHG Beamline with a raw image from an XUV spectrometer showing the high order harmonics generated by L1 laser pulses.
Members of the HHG group of the RP2 team from left Ondřej Finke, Jaroslav Nejdl, and Ondřej Hort.