**Acknowledgements**

To demonstrate the high potential of the hybrid approach relying on the optically driven broadband active media in the visible, two femtosecond hybrid systems are now under development with the aim of conducting proof‐of‐principle experiments: THL‐30 at LPI, designed for about 5 TW of output peak power, and THL‐100 at IHCE, designed to be ten times more powerful. Behind these systems is the amplification of the second harmonic of Ti:sap‐ phire front ends in the power‐boosting XeF(C‐A) amplifiers driven by the e‐beam‐to‐VUV flash converters. In the pilot experiments performed in the THL‐100 system, peak power of 14 TW has been attained in the 50 fs pulse at the output energy of 0.7 J. After upgrading pumping source, an energy output has been enhanced up to 2.5 J in the 2.4 ps pulse before its recom‐ pression promising a peak power of 50 TW to be obtained. Besides spectral matching between a solid‐state frond‐end and gas XeF(C‐A) amplifier, the nonlinear frequency upconversion results in efficient temporal cleaning of the ultrashort optical pulse, thereby providing a high contrast ratio for the output blue‐green pulses produced by a hybrid laser chain. This was confirmed by the results of ASE measurements in the XeF(C‐A) amplifier of the THL‐100 system, which argue that a contrast ratio of 1012–1013 is feasible in the blue‐green hybrid

By the example of LWFA, HHG, and recombination soft X‐ray lasers, it was shown that, in some cases, application of shorter wavelength lasers (as compared to Ti:sapphire lasers operating in the NIR) for laser‐matter interaction may be advantageous and extends the frontiers of experimental ability to provide deeper insight into the physical mechanisms of the laser‐matter interaction. One of the greatest challenges is the development of recombination‐ pumped soft X‐ray lasers that have potential to extend SXRL spectral range towards "water

Actually, the above‐discussed blue‐green hybrid concept can be considered as an alternative to the direct nonlinear upconversion of intense NIR laser radiation to the visible with the use of second harmonic generation (SHG) technique. However, to the best of our knowledge, the highest peak power reached so far in the visible with SHG does not exceed 4 TW, producing

poor beam quality [83]. Achieving higher parameters in Ti:sapphire laser systems with SHG meets serious technical problems arising from a variety of nonlinear effects in crystals at high intensities leading to a significant spatiotemporal degradation of beam quality [84]. Moreover, a broad spectrum of femtosecond pulses and strong nonlinear wave front distortion require application of very thin (0.5–1 mm) nonlinear crystals of large diameter (>10 cm). The tech‐ nology of such crystals manufacture is not yet available. Nevertheless, a large ongoing effort is presently devoted to overcome these difficulties in SHG and to reach hundreds of TW at wavelength of the second harmonic [85, 86]. The hybrid (solid/gas) laser technology is free of these problems because peak powers of 0.1–1 TW are required for a seed pulse generated by the solid‐state front end in order to extract most of the energy stored in the final gaseous

At the same time, it is necessary to say that the hybrid systems relying on the photochemically driven boosting amplifiers are inferior to the all‐solid‐state systems from the view point of a pulse‐repetition rate reaching 1 kHz at moderate output peak powers. The hybrid systems

the peak intensity in a focal spot diameter of about 3 μm as low as 3 × 1018 W/cm<sup>2</sup>

femtosecond systems with a peak power of about 100 TW.

window" and beyond.

15222 High Energy and Short Pulse Lasers

amplifier.

The work was supported by the Russian Foundation for Basic Research (Grant No. 15‐19‐ 10021).
