3.2.1

Lasing properties of J = 0-1 and J = 2-1 lines in neon-like Ge

On the CE x-ray laser scheme, we have started with the Ne-like Ge (Z = 32),²⁴ in collaboration with the group of Queen’s University Belfast (QUB), UK headed by Prof. Ciaran Lewis. Two laser beams of 1.053-μm wavelength, 1-ns pulse width and 1.1 kJ energy per beam was focused to a line of 6 cm length and 100 μm average width, with an average intensity on target of 2×10¹³ W/cm². Either an exploding foil (40-mm length), a single flat-slab (40-mm length) or a double-flat-slab target ^{25, 26} was irradiated with 2 laser beams from opposite directions (see Fig. 9). The double-slab target was composed of two slab targets of half-length (26-mm length), and each slab was irradiated with a laser beam whose half aperture was blocked. Definite amplification was observed in the five lasing lines of Ne-like Ge; 19.6 nm due to J = 0-1 transition, 23.2, 23.6 and 28.6 nm due to J = 2-1transitions, and 24.7 nm due to J = 1-1 transition.²⁷ It was found that the lasing property of the J = 0-1 line was quite different from that of the J = 2-1 and J = 1-1 lines. The J = 0-1 line was amplified in the rising part of the laser pulse, and emitted at large deflection angle of ~10 mrad from the target surface with the single slab target. These results were consistent with the prediction that the 2p⁵3p (J = 0) state, the upper level of the J = 0-1 transition, is populated by monopole collisional excitation from 2p⁶ (J = 0) ground state at high electron density, whereas the upper levels of the J = 2-1 transition are populated through relaxation from higher levels.

Figure 6.

Schematic views of (a) single flat-slab target and (b) a single curved-slab target. The approximate trajectories of the x-ray laser beam in the plasmas of these targets are shown by the solid curves.

3.2.2

Refraction compensation and double-pass amplification with a curved-slab target in an x-ray half-cavity

In order to compensate for refraction of the x-ray laser beam due to density gradient with the slab target, which became evident in these experiments, we have tested the curved slab target originally proposed by Lunny.²⁸ Figures 6(a) and 6(b) show schematic views of a single flat-slab target and a single curved-slab target, respectively. With the curved slab target, the x-ray laser beam propagates through the gain region over a long distance with a curvature which matches the refracted beam path. A Mo-Si multilayer x-ray mirror of 35 % reflectivity at 23.6 nm, fabricated at Canon Research Center, was placed at one end of the single curved-slab target, forming a half cavity for double-pass amplification in the Ge plasma.²⁹ (Similar experimental configuration with double flat-slab targets was reported from CLF.³⁰) The intensities of the J = 0-1 and J = 2-1 lines in single-pass amplification have increased ~ 10 times with the curved target compared with the flat target. When the incidence angle of the x-ray reflector was optimized in the double-pass configuration, the J = 0-1 line of less than 1-mrad divergence was stably generated, indicating the x-ray laser beam was amplified in a plasma waveguide.³¹ Although the x-ray mirror was damaged by the x-ray laser beam,³² still the feedback by the mirror was effective at least in early time of the x-ray lasing.

3.2.3

Multiple short-pulse pumping

These experiments have indicated that the J = 0-1 lasing in Ge might be enhanced if the target is irradiated with shorter-duration laser pulses. Therefore, in collaboration with Prof. Chang Hee Nam of Korea Advanced Institute of Science and Technology, we have irradiated a curved slab target by two laser pulses of 100-ps duration with 300-ps separation, at the total energy of 200 – 300 J on target with various intensity ratios of the first and second pulses. With this double-pulse pumping, the J = 0-1 line was emitted with 12 times higher peak power and reduced beam divergence compared with the 1-ns pumping, resulting in 25-times improvement in brightness.³³ The intensities of the J = 0-1 and J = 2-1 lines have increased exponentially with the increase of the first pulse intensity, indicating that the first pulse is effective in producing a pre-plasma of proper density gradient and ionization balance for efficient excitation by the second laser pulse. The x-ray laser energy has increased further by the triple-pulse pumping.³⁴ This multiple-pulse pumping method has enabled us to extend the lasing with the Ni-like ions to shorter wavelengths, as described in section 3.3.1.

3.2.4

Polarization of the x-ray laser

The soft x-ray laser generated by amplified spontaneous emission is generally considered unpolarized. As an attempt to generate a polarized x-ray laser beam, we have tested double-pass amplification in a Ge curved-slab target with a polarizing half cavity shown in Fig. 7. ³⁵ Between the Ge amplifier and a normal incidence multilayer reflector, we have placed a multilayer polarizer which had a polarization ratio of R_s/R_p = 120 for the J = 0-1 lasing line, where R_s and Rp are the reflectivities for the s- and p-polarizations, respectively. Although the total reflectivity of the polarizer and the reflector was rather small (6.3 x 10^-3 in double pass), we have obtained a polarized x-ray laser beam due to double-pass amplification with approximately the same intensity as the single-pass amplified beam, but with different emission time.

Figure 7.

A polarizing half cavity with a multilayer polarizer placed in front of the reflector, and measurement of the x-ray laser beam with a flat-field grating spectrometer and a streak camera. [Ref. 35]

The soft x-ray laser beam generated by amplified spontaneous emission might be polarized if the plasma has spatial anisotropy. In collaboration with Prof. Takashi Fujimoto of Kyoto University, we have measured the polarization of the J = 0-1 line of Ge at 19.6 nm, generated by 100-ps, double-pulse irradiation of a curved slab target. An x-ray polarizer, composed of two Mo-Si multilayer reflectors of 45-degree incidence angle with a polarization ratio of Rs/Rp > 100 each, was placed just in front of the spectrum recording plate at the output end of a high-resolution spectrometer (described in 3.2.5), and was rotated shot-by-shot to measure the intensities of the various polarization components of the x-ray laser beam. Denoting the coordinates x and z along and normal to the target surface respectively and y in the observation direction as shown in Fig. 8, it was found that the x-ray laser beam was partially polarized in the x-direction; Ix was approximately 3 times higher than I_z, where Ix and I_z are the intensities of the x- and z-polarized components of the x-ray laser beam, respectively.³⁶ Based on quantitative evaluation of the several factors that might generate polarization anisotropy, it was concluded that the observed polarization is ascribed to the anisotropy in the trapping of the resonance line 2p⁵3s (J = 1) – 2p⁶ (J = 0). The reabsorption of the resonance line emitted toward the z-direction from the dipole oscillating in the x-direction is reduced due to the velocity gradient along z. This results in the population anisotropy of the lower level of the J = 0-1 transition of n(3s)±1 < n(3s)0 where the subscripts refer to the magnetic quantum number M with z taken as the quantization axis, leading to larger population inversion for the polarized lasing in the x-direction.

Figure 8.

Configuration for polarization measurement of the x-ray laser generated in amplified spontaneous emission.

3.2.5

Linewidth of the x-ray laser

We have measured the linewidths of the lasing lines of the Ge laser using a grazing incidence spectrometer with the resolution of λ/Δλ = 16,000 (Hettrick Scientific, Model HIREFS-170.25), where a 375 lines/mm unevenly spaced grating was located at 134 cm from an entrance slit and the focal plane was located at 409 cm from the grating.³⁷ With this spectrometer, each lasing line was clearly resolved enabling determination of the linewidth of each line. The spectra were recorded with a CCD camera consisting of 1024 x 1024 array of 18-μm square pixels with a dynamic range of 10,000, which was developed for x-ray astronomy by Prof. Hiroshi Tsunemi of Osaka University.³⁸ After spectral deconvolution, the linewidths of the J = 2-1, 23.2 nm and 23.6 nm lines and the J = 0-1, 19.6 nm line were determined as 22, 21 and 25 (±4) mA, respectively.³⁹ These observed linewidths are consistent with the gain-narrowed widths of the intrinsic line broadening of ~ 50 mA due to Doppler broadening, collisional broadening and Stark broadening, calculated for the gain-length product of gL = 5-9 in this experiment. More detailed understandings on the atomic physics will be obtained by further increasing the spectral resolution.

3.2.6

In-line soft x-ray holography

A preliminary experiment of the in-line Gabor holography of simple structured objects were demonstrated using the Ge soft x-ray laser as the light source, in collaboration with Prof. T. Honda of Tokyo Institute of Technology, Prof. Kunio Shinohara of Tokyo Metropolitan Institute of Medical Science, Prof. David Neely and his colleagues at QUB and King’s College London. The holograms recorded on PMMA were retrieved with an atomic force microscope. The object image was reconstructed with phase retrieval algorithm, resulting in a clear, ghost-free image of sub-μm resolution, showing the potential of the x-ray laser for high-resolution 2-D and 3-D imaging. ⁴⁰

3.3.1

Lasing in Ni-like ions at 14.3 - 6.37 nm by multiple-pulse pumping

By applying the triple-pulse pumping technique^{33, 34} to the Ni-like x-ray laser, we have succeeded in demonstrating lasing over the wavelengths of 14.3 – 6.37 nm in the following Ni-like ions; ₄₇Ag (λ = 14.3 nm), ₅₂Te (11.1 nm), ₅₇La (8.9 nm), ₅₈Ce (8.6 nm), ₆₀Nd (7.97 nm), ₆₂Sm (7.32 nm), ₆₄Gd (6.92 nm), ₆₅Tb (6.67 nm), and ₆₆Dy (6.37 nm), where the wavelengths of the observed lasing lines of the long wavelength component are shown in the parentheses.^{41, 42} In these experiments, a curved slab target was irradiated with either double, triple or quadruple 1.053-μm laser pulses of 100-ps duration separated by 400 ps, with the total energy of 200 - 500 J. A gain coefficient of g ~ 3.1 cm^-1 and a gain length product of gL ~ 8 have been achieved at 7.97 nm in the Nd ions. This multiple-pumping method has proven to be effective for reducing the laser energy required to pump these soft x-ray lasers by almost an order of magnitude. This work was later extended to the demonstration of the saturated gain in Ni-like Ag at 13.9 nm and Sn at 12.0 nm by 4-ps duration, 14-J, two-pulse pumping with a table-top laser at the JAERI Advanced Photon Research Center.⁴³

3.3.2

Intense lasing in Nd and Ag by quasi-travelling-wave pumping of double curved-slab target

In collaboration with the group of National Laboratory on High Power Lasers and Physics (NLHPLP), Shanghai Institute of Optics headed by Prof. Shiji Wang, where double-target configuration has been tested in 1992,²⁵ we have irradiated two curved-slab targets by two laser pulses with a 100-ps delay for quasi-travelling-wave pumping of the two targets, as shown in Fig. 9. Double-pass amplification using a 100-layer-pair Ru/B₄C multilayer mirror fabricated at NTT-Advanced Technology was also implemented, where the reflected x-ray laser pulse was amplified in the plasma pumped by the next laser pulse. For target irradiation, instead of using a standard single cylindrical lens, a novel optics composed of 4 convex cylindrical lenses developed at NLHPLP was installed for uniform-width line focusing.⁴⁴ A large-aperture deformable mirror was also tested for uniform line focusing, resulting in improved soft x-ray laser performance.⁴⁵

With the single-pass quasi-travelling-wave amplification, we have obtained very intense lasing in the Ni-like Nd at 7.97 nm with ~ 40-ps duration and ~ 40-μJ energy corresponding to ~ 1-MW peak power,⁴⁶ and saturated amplification in Ni-like Ag at 13.9 nm with an exceptionally high gain of 19 cm^-1, ~ 300-μJ energy and 5 MW peak power.⁴⁷ In this experiment, we found that the merit of the curved target decreased in the multiple pulse pumping scheme as discussed by Nilsen et al.⁴⁸

Figure 9.

Quasi-travelling wave pumping of a double slab target.

3.3.3

Extension of lasing close to the water window

For x-ray lasing close to the water window with the Ni-like ions, a high density and high temperature plasma is required as an amplifying medium. For example, the desirable electron temperature and the electron density are 1 – 1.5 keV and 1.5 x 10²¹ cm^-3 for ₇₀Yb and 1.4 – 1.8 keV and 3.5 x 10²¹ cm^-3 for 74W, respectively.⁴⁹ In this case, the important issue is the propagation of the x-ray laser beam through the gain region in the plasma.⁵⁰ The x-ray propagation through the gain region is quite different at the wavelengths shorter than 10 nm, where the obliquely incident x-ray laser beam penetrates in the high-density non-amplifying plasma and absorbed. In this spectral region the curved target is not effective for refraction compensation, because the critical density at 10 nm is ~1×10²⁵ cm^-3 which is much higher than the solid density plasma.

Experiment on lasing toward the water window in the Ni-like ions was implemented in collaboration with NLHPLP and the group of Friedrich-Schiller University Jena headed by Prof. Eckart Förster. Two flat-slab targets of 8 mm length each was irradiated by two counter-propagating lasers of 1.053-μm wavelength and 100-ps duration with a time delay for quasi-travelling wave pumping. Each laser beam was composed of a pre-pulse and a main pulse, with the main-pulse energy of 240 J and intensity of 3 x 10¹⁴ W/cm². After extensive optimization of the target position (target separation along and vertical to the x-ray laser beam axis) and laser parameters (delay time and pre-/main-pulse intensity ratio), the x-ray lasing near the water window were obtained in Ni-like Yb, Hf and Ta ions at 5.0 nm, 4.7 nm and 4.5 nm, respectively as shown in Fig. 10. The gain coefficients of the Ni-like Yb and Hf lasing lines were g = 6.6 cm^-1 and 3.6 cm^-1 with the gain length product of gL = 11 and 6, respectively.⁵¹ Indication of lasing in W at 4.3 nm, in the water window, was also observed. According to 1-D hydrodynamic simulation for 1-μm laser irradiation, the electrons are heated to ~ 1 kV, but the electron density of the heated region of ~ 10²¹ cm^-3 is lower than the required density.

Figure 10.

Observed spectra of lasing in (a) Yb, (b) Hf and (c) Ta, respectively. The carbon K-edge at 4.38 nm is shown as the edge of the water window. [Ref. 51]

3.3.4

Spectroscopic study of the lasing lines in the nickel-like ions

Although the observed intensities of the Yb, Hf and Ta x-ray lasers were below the saturated levels, the well-defined lasing lines are precious for spectroscopic studies. Comparing the observed wavelengths and intensities of the lasing lines in Ni-like ₆₀Nd, ₆₂Sm, ₆₄Gd, ₆₆Dy, ₆₇Ho, ₇₀Yb, ₇₂Hf, and ₇₃Ta, we found that the intensity of the longer wavelength component is stronger in ₆₀Nd ~ ₆₇Ho, whereas the intensity of the shorter wavelength component becomes stronger than the longer wavelength component in ₇₀Yb ~ ₇₃Ta. Prof. Fumihiro Koike of Kitasato University has successfully clarified these experimental results of the wavelengths and line intensities, by implementing detailed relativistic atomic-physics calculation on these elements to derive the wavelengths and the oscillator strengths and also solving a simplified rate equation code to calculate the relative gain of these lines using the calculated oscillator strengths. ⁵²

Detailed account of the experiments on the development and application of the collisional-excitation soft x-ray lasers at ILE can be also found in the review papers. ^{49, 53-55}