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The generation of high-brightness plasma-based soft X-ray lasers at the laboratory scale is of high interest for numerous applications. One of the limitations has been the repetition rate of the laser driver, which is used to create the amplifier plasma. Collisionally pumped soft X-ray lasers in the 10- to 30-nm range operate at 10 Hz for more than 5 years [1], [2], [3], [4]. However, recent achievement has increased the repetition rate by an order of magnitude; a soft X-ray laser at 100-Hz repetition rate with a record-breaking average power of 0.15 mW at Formula$\lambda = 18.9\ \hbox{nm}$ has been demonstrated in 2012 [5]. These results open the path of multimilliwatt average power soft X-ray beams on a table top.

Several applications require shorter wavelength soft X-ray lasers, such as the so-called “water window” Formula$(\lambda = 2.2{-}4.4\ \hbox{nm})$ that is important for biological imaging. Although the shortest lasing wavelength demonstrated to date is 3.5 nm, soft X-ray lasers emitting below 10 nm were limited to kilojoule-class driving lasers operating at very low repetition rate (1 shot/hour or less), hence severely limiting their use for applications. Important progress has been achieved in 2012 by demonstrating lasing at wavelengths significantly below 10 nm with table-top 10-J laser systems operating at up to 1 Hz. This was obtained by scaling collisional pumping of nickel-like ions toward higher atomic numbers with an increased pumping laser power. Using a 10-TW Nd:glass, chirped-pulse amplification (CPA) driving laser and the grazing-incidence pumping geometry, gain-saturated lasing was obtained at 9.2 nm in barium [6], while strong lasing was recently observed in samarium at 7.36 nm [7]. Using a 7.5-J Ti:Sapphire CPA laser operated at 1 Hz, efficient generation of gain-saturated soft X-ray laser emission with up to 2.7-Formula$\mu\hbox{J}$ energy was demonstrated at 8.8 nm in Ni-like lanthanum [8]. In the same experiment, isoelectronic scaling along the lanthanide series resulted in lasing at several wavelengths down to 7.36 nm.

The only way to progress toward significantly shorter wavelengths, namely, below 2 nm, is to investigate different pumping schemes involving lasing levels with a larger energy gap. Recent experiments performed at the LCLS X-ray free-electron laser have demonstrated a strong inner-shell photopumping of atoms by intense femtosecond burst of X-rays. By rapid K-shell photoionization of a dense volume of neon gas, a transient population inversion in neon has been established, resulting in the saturated amplification of the K-alpha line at 1.5 nm along the XFEL pump pulse [9] [see Fig. 1(a)].

Figure 1
Fig. 1. (a) Experimental set-up and recorded spectrum showing the atomic X-ray laser line photopumped by the XFEL pulse [9]. (b) Sequential images of an oscillating nanoprobe sampled by the soft X-ray laser pulse [19]. (c) Artistic view of an amplification chain proposed to boost the soft X-ray laser output power and to reduce the pulse duration below 100 fs [25].

Plasma-based soft X-ray lasers operated in the single-pass amplification of spontaneous emission (ASE) mode have now reached a high level of maturity allowing for the development of a variety of applications at the laboratory scale. Dedicated soft X-ray laser beamlines are already available for users in several laboratories worldwide, particularly at CSU (USA) [10], APRC-JAEA (Japan) [11], PALS (Czech Republic) [12], and LASERIX (France) [13]. Innovative techniques and diagnostics relevant to topical domains such as EUV lithography, inertial confinement fusion, or warm dense matter physics have been developed. The technique of Talbot self-imaging of a periodic mask was extended to the 50-nm spectral range to demonstrate defect-free patterning of nanoscale features on a photoresist [14]. A 13.9-nm laser was used to obtain single-shot coherent diffraction imaging with 100-nm spatial and picosecond temporal resolutions [15]. The processes of ablation and damage-threshold of different metallic and nonmetal materials with a focused soft X-ray laser were investigated [16]. The strong variation of EUV opacity of an iron foil through ionization was used to investigate the energy transport in a short-pulse-laser-heated target on a picosecond timescale [17]. The 21.2-nm laser was used to sample the propagation of a laser-induced radiative shock wave in a xenon gas [18]. New prospects in the field of nanoelectromechanical devices have also been demonstrated with the visualization of rapid dynamic interactions in the oscillating movement of a magnetic nanoprobe by flash-soft X-ray microscopy [19] [see Fig. 1(b)]. Finally, the technique of laser ablation mass spectrometry was extended to the soft X-ray domain to allow chemical nanoprobing of irradiated surfaces with high sensitivity [20].

Although the potential for applications of plasma-based soft X-ray lasers is constantly developing, the emergence of X-ray free-electron lasers changed the horizons: physics at ultra-high-intensity single-shot imaging at nanometer scale are some of the new fast evolving fields. In this race for ultra-high intensity and ultra-fast X-ray pulses, plasma-based soft X-ray lasers operated in the ASE regime must overcome their current limitations, particularly a shortest pulse duration limited to about 1 ps, which, in turn, limits the peak power to few MW (GW for the X-FEL). Seeding the amplifying plasma with a femtosecond high-order harmonic of infrared laser was foreseen as the required breakthrough to break the picosecond frontier. Experiments in both gas and solid amplifiers [21], [22] succeeded in reducing the pulse duration to values as low as 1 ps but failed to reach the femtosecond range, as well as to demonstrate a significant level of amplified seed energy over the ASE background. This is mainly related to the extremely narrow gain bandwidth of Formula$\sim\! \!10^{-4}{-}10^{-5}$ typical of collisionally pumped soft X-ray lasers, as was extensively investigated recently [23], [24]. Numerical codes based on Maxwell–Bloch model were developed to explore this new regime of amplification. Recent simulations propose new promising paths toward substantially shorter duration and increased amplified energy [25], [26], notably transposing the Chirped Pulse Amplification technique to the X-ray domain [26]; in order to increase the time over which the femtosecond seed can be amplified by the picosecond high-gain amplifier, the high-order harmonic pulse is first stretched by a pair of gratings and then coherently amplified to extract the large amount of energy stored in the plasma amplifier [see Fig. 1(c)]. The amplified pulse is finally recompressed by a second pair of gratings. Detailed simulations predict that such a configuration should allow to reaching an amplified energy as high as 6 mJ in 230-fs duration, corresponding to the outstanding power of 20 GW. With the state-of-the-art Ti:Sa laser drivers, such a high-power soft X-ray laser beam would be operated at one shot per minute, but extension to 1 Hz can be anticipated in the forthcoming years owing to the fast progress in diode-pumped high-power lasers.

Footnotes

Corresponding author: A. Klisnick (e-mail: annie.klisnick@u-psud.fr).

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S. Sebban

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Ph. Zeitoun

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A. Klisnick

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