High-order harmonic generation (HHG) is a strongly nonlinear process in which femtosecond (1 fs = 10-15s) laser pulses are disturbed by a medium, e.g. gas, in such a way so that very high-order harmonics of fs laser wavelength are created. Typically, the harmonics orders could as big as 27th and higher. In physical explanation the phenomenon involves outer most electrons of the (for example) gas atom. The atom is ionized by very strong laser field. The resuming energy of the laser field is then transferred into kinetic energy of the ionized electron. After field sign reversal the electron is decelerated and finally stops. Then, it is accelerated back towards the atom and re-collides. During recombination the excess of kinetic energy of captured electron is emitted as a photon which energy is proportional to kinetic energy of the electron. The description above is known as so called “three step model” (Fig. 1).
Fig. 1. Summary of the 3-step model of HHG.
High-order harmonic sources feature unique properties: they are highly coherent, radiation is emitted in a beam-like shaped cone, beam profile is well defined (almost perfect Gaussian intensity distribution) and of very low divergence (below mrad), in spectral domain the source results in a comb of extremely narrow lines. They are also tunable. This type of source cannot provide with very intense beams but they are rather complementary sources to very strong low-repetition rate X-ray lasers since their repetition rate may be as high as 10 Hz to 1 kHz (typically). High-order harmonics, though, are so far the only method to generate light pulses in range of attoseconds (1 as = 10-18s) – the shortest light bursts ever obtained.
Our group has developed, characterized, optimized and comissioned an ultrafast coherent X-ray beamline at PALS. The beamline is based on 1 kHz, table-top, HHG source capable to deliver fully coherent beam in the spectral range 20 - 35 nm. Ti:sapphire (810 nm) laser pulses with a duration of 35 fs and energy 1.2 mJ (Fig. 2)are focused into low pressure static gas cell or high density pulsed gas jet containing conversion medium (Ar).
Fig. 2. Laser system used for HHG at IoP/PALS.
The experimental intrastructure for HHG is shown in Fig. 3.
Fig. 3. HHG experimental beamline at IoP/PALS. The beamline comprises titanium-doped sapphire laser system, HHG source placed in the source vacuum chamber (the blue in the picture) and application chamber (grey in the picture, downstream beam propagation direction).
The experimental setup is shown in Fig. 4.
Fig. 4. Schematic experimental drawing of HHG source at the IoP/PALS.
The source has been characterized with respect to its properties. Some experimental results are presented in the Fig. 5 and Fig. 6.
Fig. 5. High-order harmonic beam footprint.
Fig. 6. High-order harmonic spectrum from Ar. Raw data (upper part) and wavelength-calibrated graph (lower part).
The developed strong HHG source is now being employed in various interdisciplinary applications such as X-ray ablation and metrology of X-ray optics.
The high-order harmonic source was used for reflectivity measurements of multilayer mirror (ML) versus incidence angle. The experimental setup is presented in Fig. 7 and Fig. 8.
Fig. 7. Experimental setup of measurement of reflectivity of multilayer mirror versus incidence angle. Experiment was performed at KAIST.
Fig. 8. Photo of the experimental setup.
The mirror was placed on the rotational stage together with the XUV sensitive absolutely calibrated photodiode allowing change of the incidence angle under vacuum. The photodiode is additionally covered with 150 nm Al filter. The signal was acquired with an oscilloscope. Experimental results are presented in Fig. 9. The measured and simulated reflectivity for the mirror are presented. The measured reflectivity was 31.5 % (fitted value) and simulated ~29 %.
Fig. 9. Reflectivity measurement of Mo/Si flat multilayer mirror designed for 21.6 nm radiation incident at 45° angle.
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