We report on recent progress achieved in X-ray laser research at the Institute of Applied Physics of the University of Bern. Using the existing 10-TW Nd:glass CPA (chirped-pulse amplification) laser system in the grazing-incidence pumping (GRIP) scheme, saturated or near-saturated soft-X-ray lasing has been obtained on the 4d→4p, J = 0-1 lines of barium (Ba, Z = 56), lanthanum (La, Z = 57), and samarium (Sm, Z = 62) at wavelengths down to 7.36 nm, with weak lasing observed at 6.85 nm in Sm. This was achieved with main pulse energies of ~10 J at a pulse duration of 1.5 ps. A small-signal gain coefficient of ~30 cm<sup>-1</sup> and a gain-length product of ~16 at saturation have been measured in the case of the 9.2-nm Ba laser. Crucial to these results was the introduction of a second, relatively intense (~20%) prepulse less than 50 ps before the main pulse, in addition to the 2.8% prepulse that irradiated the target ~3 ns earlier. Travelling-wave excitation was used throughout.
Extreme ultraviolet (EUV) lasers in the wavelength range of ∼ 10 to 20 nm have matured to a point where dedicated applications such as at-wavelength inspection of extreme-ultraviolet lithography (EUVL) masks become possible on the laboratory scale. The authors briefly review the principles of plasma-based EUV lasers, the progress made so far, and the output characteristics of interest to the EUVL community.
High brightness extreme ultraviolet (EUV) light sources for laboratory operation are needed in nano-fabrication and
actinic ("at-wavelength") mask inspection. Mask inspection in next generation lithography is crucial for high volume
manufacturing. Plasma-based EUV sources have the required compactness. However, their incoherent emission
lacks the brightness for fast and high contrast imaging. The X-ray laser is instead characterized by a remarkable
brightness in a compact footprint facility. We evaluated a simple two-mirror optical setup for EUV microscopy
illuminated with the BeAGLE X-ray laser system at the University of Berne. Single-shot acquisitions were sufficient
to obtain high-contrast images of a Siemens star sample at diffraction-limit. Single-shot operation makes the overall
acquisition speed limited by the laser repetition rate only. A reference calculation shows how-fast could be actinic
inspection. The contrast was enhanced one order of magnitude by means of image processing. For a modest
magnification (12x) no significant third-order aberrations were observed, even when tilting the spherical mirror-pair.
For high magnification a Schwarzschild design is considered. The latter compensates astigmatism and coma with a
mirror-pair per each element (condenser/magnifier), but introduces twice as many reflections as in the evaluated
two-concave setup. Hence a compromise between aberration correction and enhancement of illumination must be
found case by case.
The development of plasma excited X-ray lasers is of interest for many scientific applications. The photon energies and
peak brilliance of these lasers sources are well suited for probing atomic, molecular and solid state systems. The
development and improvement in these laser systems also drives a need for metrologies of the properties of these lasers.
Our research implements X-ray optics, designed to operate at the Brewster's angle, to measure the polarization state of a
Ni-like Sn laser. The device determines the polarization state on a shot to shot basis and opens the possibility for
polarization control of plasma excited X-ray lasers and thus probing spin polarized electronic states.
We report on recent progress achieved in x-ray laser research at the University of Bern. Using the existing 10-TW
Nd:glass CPA (chirped-pulse amplification) laser system in the grazing-incidence pumping (GRIP) scheme, saturated x-ray
lasing is demonstrated on the 4d → 4p, J = 0-1 line of Ba at a wavelength of 9.2 nm, using a main pulse energy of
9 J. A small-signal gain coefficient of ~30 cm<sup>-1</sup> and a gain-length product of ~16 at saturation have been measured.
Crucial to these results was the introduction of a second, relatively intense (~10%) prepulse less than ~100 ps before the
1.5-ps duration main pulse, in addition to the 2.8% prepulse that irradiated the target 2.4 ns earlier. Travelling-wave
excitation was used throughout. For handling convenience, compound targets (BaF<sub>2</sub>, LaF<sub>3</sub>) were used, either in the form
of windows or coated onto glass slides.
We report on gain-saturated operation of the 4d → 4p, J = 0-1, 11.4-nm soft-x-ray laser line in Ni-like antimony (Sb) at a pump energy of only 2.5 J. The driving laser used was a 1054-nm Nd:glass CPA laser system with a pulse duration of 7 ps (FWHM). The pump beam was focused with a tilted on-axis parabolic mirror in a grazing-incidence (GRIP) pumping configuration at a grazing angle of 45°. For this angle, the length of the line focus attains a value of 19.6 mm (FWHM) for the 120-mm beam diameter used in these experiments. In addition, the excitation velocity of the intrinsic
traveling wave increases to 1.41c and results in a temporal mismatch of ~20 ps at the output end of the target. As a consequence, care must be taken in interpreting the usual gain measurements
(intensity-vs.-target length). A novel method to distinguish the different effects causing roll-off based on the second derivative of the intensity curve will be discussed.
We report on gain-saturated operation of the 11.9-nm x-ray lasing line in Ni-like Sn using the grazing-incidence pumping
scheme (GRIP). The experiments were done with 2-ps duration pump pulses and energies up to 5 J. Strong gain saturation
with ⪆10-microJ output was measured for the Sn laser at a grazing angle of 45° and a pump pulse energy of 5 J. This
was achieved with a 4.5%, 2-ps duration prepulse 1.6 ns ahead of the main pulse and also incident at grazing incidence.
Increasing laser output was observed at GRIP angles from 22.5° to 45°. At this angle, the minimum energy required for
saturated lasing was determined as ~2 J.
The absolute time of emission of the x-ray laser output with respect to the arrival of a 100-ps pump pulse has been measured with the aid of a calibrated timing fiducial. The results show the x-ray laser to appear up to 60 ps (80 ps) before the peak of the pump pulse in the case of the Sn (Pd) x-ray laser, which is in good agreement with results obtained from hydrodynamic, atomic physics, and raytracing simulations. The pulse duration was found to be ~40 ps for both the Sn and the Pd x-ray lasers.
We report on 2D near-field imaging experiments of the 11.9-nm Sn x-ray laser that were performed with a set of novel Mo/Y multilayer mirrors having reflectivities of up to 40% at normal and at 45° incidence. Second-moment analysis of the x-ray laser emission was used to determine values of the c-ray beam propagation factor M2 for a range of irradiation parameters. The results reveal a reduction of M2 with increasing prepulse intensity. The spatial size of the output is a factor of ~2 smaller than previously measured for the Pd x-ray laser, while the distance of the x-ray emission with respect to the target surface remains roughly the same.
We report on the results of a series of experiments aimed at the full characterization of gain-saturated nickel-like soft x-ray lasers at wavelengths between 14.7 nm and 12.0 nm. The experiments include measurements of the gain-length product, output energy, near-field and far-field intensity distributions, and coherence properties of Pd and Sn x-ray lasers. Most of the experiments were done at a drive irradiance of 10 TW/cm<SUP>2</SUP>, using a 1054-nm drive energy of 30 J in 100-ps pulses.
We report on the experimental demonstration of saturated x- ray laser output from collisionally pumped Ne-like Fe at 25.5 nm as well as Ni-like Ag and Pd at 14.0 nm and 14.7 nm, respectively, using a 100-ps drive pulse irradiation. A double-prepulse scheme and a 3-m radius-of-curvature target resulted in a gain-length product of gL equals 16.5 in the case of Fe. With a single prepulse and flat slab targets, gain- length products of 15.3 and 15.8 were obtained for Ag and Pd, respectively. Saturation was also confirmed by the observed reduction in beam divergence with increasing target length. The required drive energy used was only 30 J in a 100-ps pulse, corresponding to an irradiance of 12 TW/cm<SUP>2</SUP>. A key role in the achievement of these results was played by the reduction in the roughness of the target surface in the case of the Pd x-ray laser.
Using the prepulse technique, soft-x-ray lasing is demonstrated on the J equals 0 - 1 transitions of neon-like titanium, chromium, and iron at 32.6, 28.5, and 25.5 nm, respectively, and on the J equals 2 - 1 transitions of copper, zinc, and germanium at 27.9/28.5, 26.2/26.7, and 23.2/23.6 nm, respectively, for pump energies below 100 J, corresponding to irradiances of less than 10 TW/cm<SUP>2</SUP>. A gain coefficient g equals 2.2 cm<SUP>-1</SUP> was measured for the 28.5-nm line of Cr at a pump energy of 90 plus or minus 5 J and with a 0.7% prepulse preceding the main pulse by 5 ns, resulting in a gain-length product of approximately 5.5. Angle-resolved spectra indicate a beam divergence of 3 mrad and a beam deflection angle of approximately 6 mrad. The space-resolved spectra show that the J equals 0 - 1 lasing line is emitted from an approximately 60-micrometer wide plasma region, while the nearby continuum emission is produced in a considerably broader plasma region of approximately 230 micrometer. Lasing was observed at pump energies as low as 60 J (approximately 6 TW/cm<SUP>2</SUP>), however with considerable shot-to-shot scatter in the data.
The laser performance of Nd:KGW (neodymium-doped potassium gadolinium tungstate) end pumped by cw or quasi-cw high-power diode laser bars is studied and compared to Nd:YAG. In cw operation, thermal problems in Nd:KGW are quite important, whereas in quasi-cw operation, the optical and slope efficiencies attained with Nd:KGW are clearly better than with Nd:YAG.