Laboratoire d'Optique Appliquée


Dec 09 2021


10 h 00 min

Physics and applications of a coherent femtosecond source at 32.8 nm

We report here recent work on an optical-field ionized EUV laser at 32.8 nm, which is seeded by an external high order harmonic source allowing to improve the spatial properties of the EUV laser while exploiting the energetic qualities of the plasma amplifier.

The gain lifetime of the EUV laser amplifier strongly depends on the depletion rate of the lasing ion population. Indeed, electron collisions with the lasing ions contribute to build up a population inversion but also lead to the overionization of this ion, and therefore to an anticipated interruption of the gain. When increasing the plasma density from 3 × 1018 cm−3 up to 1.2 × 1020 cm−3, the gain duration monotonically decreases from 7 ps to an unprecedented shortness of 450 fs FWHM [1]. The EUV pulse duration being dictated by the duration of the gain, we newly implemented a single shot diagnostic allowing to measure the temporal profile of the EUV laser and observed a reduction of the pulse duration with respect to the electronic density down to 900 fs FWHM at 7.6 × 1019 cm−3.

Measurements performed with this diagnostic, in good agreement with simulations, have also shown that the EUV laser pulse duration can be controlled in a wide range by ajusting the amount of spatio-temporal couplings in the pump beam [2], and therefore regulating its group velocity [3]. Pulse duration as short as 450fs FWHM has been obtained with this technique.
Finally, we demonstrated second harmonic generation (SHG) on a table-top XUV source for the first time by observing SHG at the Ti M2,3-edge with a high harmonic seeded soft x-ray laser[4].

[1] : A. Depresseux et al. Nat. Photonics, Vol. 9, 817 (2015)
[2] : A. Kabacinski et al. Journal of Optics, Vol. 23, 06LT01 (2021)
[3] : A. Sainte-Marie et al. Optica, Vol. 4, 1298 (2017)
[4] : T. Helk et al. , Science Advances, Vol. 7, eabe2265 (2021)

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