This type of short-wavelength source is based on stimulated emission similarly to other type of lasers. However, unlike in the case of lasers emitting in the visible or infrared spectral range, highly ionized plasma column forms an active medium of those devices. In this particular case the infrared Ti:sapphire laser was used to generate Mo plasma column of appropriate ionization state (Mo14+) and heat the free electrons to excite the ions thus creating population inversion – a necessary condition for lasing. Using the method presented in the paper one can estimate the pulse duration of the source from the histogram of its far-field intensity distribution when knowing the coherence properties of the light. Those can be obtained either from another measurement or from numerical simulations.
Another two published papers were related to high-order harmonic generation from noble gas. This source of coherent extreme ultraviolet radiation with femtosecond pulse duration is based on extremely nonlinear interaction of atoms with strong laser field converting very small energy fraction of the laser pulse into the burst of attosecond XUV pulses. The articles report on correlation of spectral changes of the driving laser after propagation through the generating gas with the overall efficiency of generation [2] (Applied Physics Letters) and with the spectral features of the generated XUV radiation [3] (Physical Review A). The first study concludes that one of the most critical aspect of efficient XUV generation in this kind of experiments is proper state of ionization, which is directly linked to proper laser intensity. As the laser ionizes the medium it changes its refractive index in time which causes spectral shift of the laser radiation during propagation in the medium. Observing the laser spectrum after generation allows for immediate understanding of the interaction regime and thus leads to overall better understanding and control of the generation process.
All three sets of data were acquired during two experimental campaigns using Ti:sapphire laser system at PALS in 2015 and 2016. „The publications went out pretty fast, but it took us long time to elaborate the physical models, which are able to explain reliably the physics. We hope it will be of much use for the community developing laser-driven short-wavelength sources,“ says Jaroslav Nejdl, the principal investigator of these research projects.
[1] M. Albrecht et al. Pulse duration of a partially coherent soft X-ray laser estimated from far-field speckle statistics, Opt. Lett. 43, 4586 (2018); https://doi.org/10.1364/OL.43.004586
[2] V. E. Nefedova et al. Efficiency control of high-order harmonic generation in gases using driving pulse spectral features, Appl. Phys. Lett. 113, 191101 (2018); https://doi.org/10.1063/1.5050691
[3] V. E. Nefedova et al., Determination of the spectral variation origin in high-order harmonic generation in noble gases, Phys. Rev. A 98, 033414 (2018); https://doi.org/10.1103/PhysRevA.98.033414
Fig. 1 Spectrum of the Ni-like Mo X-ray laser with dominant line at 18.9 nm (solid black line). Inset: typical speckle pattern in the far-field and its intensity statistics (histogram).
Fig. 2 Experimental setup of typical experiment on high-order harmonic generation with simultaneous observation of spectrum of generated XUV radiation and spectrum of the driving laser after the interaction with generating gas.
