Abstract:In this review we concentrate our attention on X-ray laser research and development covering the so called soft Xray region (down to ∼ 10 nm) and the "water window" region (down to 2.3 -4.4 nm). We present the development of soft X-ray lasers (SXLs) and their applications using primarily collisional and recombination schemes. We present the paths towards compact systems and major achievements to date. We discuss conditions for using both schemes to reach the "water window" region. In this discussion we also present another possible schemes based on photo-pumping, inner shell transitions in atoms and ions and "Doppler Compression" of IR laser radiation down to the X-ray region with its intrinsic tunability possibility. A significant part of the review is dedicated to the wide range of applications of SXLs with wavelengths at and above 10 nm. Also discussed is the potential of extending the range of biological applications for the "water window" region.Image of wafer with polysilicon lines on silicon (top: 250 nm, bottom: 100 nm widths of lines)

We report the first saturated amplification of an optical-field-ionization soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 330-mJ, 35-fs, 10-Hz Ti:sapphire laser system in a few-mm cell filled with xenon. A gain of 67 cm(-1) on the 4d(9)5p-4d(9)5d transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about 5 x 10(9) photons per pulse. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

Amplification of spontaneous emission (ASE) at 23.6 nm has been studied in a Ge plasma heated by a 1 TW infrared laser pulse. The exponent of the axial gain reached 21 in a geometry with Fresnel number < 1. Two plasma columns of combined length up to 36 mm were used with an extreme ultraviolet mirror giving double-pass amplification. Saturation of the ASE output was observed. The beam divergence was about 8x diffraction limited with a brightness estimated at 10 14 Wcm~2sr _1 . The feedback from the mirror was significantly reduced probably by radiation damage from the plasma.

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It is shown that the 2p 5 4d *P t -+ 2p Q % line of the Al 3+ -i -tp o 0 XJLUC UJL wic i-i-i. ion at a wavelength of 117.41 A is superradiant in a dense laser-produced plasma. The measured gain in a two-plasma experiment is 17%. The gain coefficient is of the order of 10 cm" 1 with a lower limit of 2.5 cm" 1 and an upper limit of 22 cm" 1 .The possibility of producing population inversions between atomic or ionic levels of convenient energy is a key point for investigating the feasibility of an x-ray laser. Several approaches have been proposed. 1 " 13 Previously we suggested that the surprisingly high intensity of the almost forbidden 2p 5 4d 3 P 1 * 2p 61 S 0 line of the Al 3 * ion in a dense laser plasma is due to the enhancement of the z P y upper-level population compared with the equilibrium value expected from Boltzmann's law. 2,3 This assumption was supported by opticalthickness measurements exhibiting the large transparency of the plasma for this line, located at a wavelength of 117.41 A, whereas the weak lines corresponding to the transitions 2p 5 4td 1 P 19 3 D^ 2p 61 S 0 (X = 116.46 and 116.92 A) were optically thick. Moreover, from time-resolved measurements in plasmas produced by 100-MW, 40-nsec Nd-laser pulses, it was concluded 14 that the 117.41-A line exhibits an emission significantly shorter than other lines of the same ion. The possibility that a small population inversion occurs was suggested by these first results, 8 ' 14,15 and a recombination mechanism involving autoionizing states of the Al 3+ ion was proposed to explain the anomalous population of the 3 P 2 level. 8,16,17 Although calculation predicted 16 that this mechanism could be effective in Ne-like ions up to Z = 18 (Ar 8 + ), experimental observations concern at present only the Al 3+ ion. Recently the possible occurrence of population inversion between excited and ground states in dense hot plasmas has been discussed for the case of hydrogenie ions. 18 Here we report new experimental results leading to the conclusion that the 117.41-A line is an actual superradiant line because it exhibits a negative absorption in the dense part of a laser plasma.Figure 1(c) shows the special feature of the Al 3+ lines (116.46, 116.92, and 117.41 A wavelength) in the plasma zone under investigation. For comparison, Fig. 1(a) represents the relative intensities as observed in the low-density region of the plasma, far from the target. Similar ratios have been observed in spark discharges 19 ' 20 showing an intensity of the 3 P X line 1 order of magnitude less than that for both other lines. The radiative-transition probabilities, calculated in J x -j coupling for the initial state and L-S coupling for the ground state 2^6 1 S 0 , are plotted in Fig. 1(b). These values are used because the agreement with experimental results in plasmas of low density is better than that obtained with values calculated in intermediate coupling 21 with Hartree-Fock wave functions. This fact, already mentioned, 16 seems related to an underestima-J A 21 10 5 D 0,5 (1...

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