**1. Introduction**

Significant progress in the development of all‐solid‐state femtosecond laser systems relying on chirped‐pulse amplification (CPA) technique has resulted in reaching petawatt (PW) peak powers [1] and focused intensities as high as 10<sup>22</sup> W/cm<sup>2</sup> [2] that provides great opportuni‐ ties for experimental studies in the area of the extreme high‐field science. The all‐solid‐state

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laser systems providing pulses shorter than 100 fs are based on the Ti:sapphire and optical parametric chirped‐pulse amplification (OPCPA) technologies.

Presented in this chapter are milestones and main results obtained in the course of the realization of a novel hybrid (solid/gas) approach to the development of femtosecond systems that, unlike the all‐solid‐state systems operating in the near‐infrared (NIR) region, allow for producing super‐intense optical pulses in the blue‐green spectral range. This approach aims to marry robust solid‐state laser technologies highly developed for femtosecond pulse generation and amplification in the NIR spectral range with advantages of photochemically driven gaseous gain media of the visible range.

Most extensive development of the photochemical method of pumping gaseous active media dates back to the 1960s–1990s of the last century (see [3–5] and references cited therein). Being applied to optical excitation of gas lasers on electronic molecular transitions by radiation from such unconventional pump sources as high‐temperature electrical discharges and strong shock waves in gas, This method resulted in emerging a new class of gaseous active media for lasers emitting in the spectral range extending from the NIR to UV with a high output energy increasing in proportion to the active volume and pump energy. Among a variety of molecules lasing upon optical excitation, there are three broadband active media (XeF(C‐A), Kr2F, and Xe2Cl), which offer a number of characteristics extremely attractive for the amplification of femtosecond optical pulses up to ultrahigh peak powers. The gaseous nature and visible spectrum of emission of these media promise important virtues of the hybrid systems. First of all, the gaseous nature of these media is characterized by low nonlinear refractive index allowing amplification of optical pulses with much higher intensities as compared with solid media. Secondly, the visible spectrum of emission requires nonlinear frequency upconversion of a seed pulse generated in the NIR spectral region by a solid‐state front end, thereby providing efficient temporal cleaning of the ultrashort optical pulse and the high temporal contrast ratio of an output pulse, which is of primary importance for a number of high‐field experiments.

The main motivation for the development of hybrid systems in the visible is favorable drive frequency scaling of laser‐matter interaction in a number of high‐field applications. Of overriding importance is the dramatic improvement of the recombination soft X‐ray laser excitation in an optical field ionized (OFI) plasma with drive frequency.
