This PDF file contains the front matter associated with SPIE Proceedings Volume 8140, including the Title Page, Copyright Information, Table of Contents, Introduction, and the Conference Committee listing.

Nearly four decades ago H-like and He-like resonantly photo-pumped laser schemes were proposed for producing X-ray lasers. However, demonstrating these schemes in the laboratory has proved to be elusive because of the difficulty of finding a strong resonant pump line. With the advent of the X-ray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source (LCLS) we now have a tunable X-ray laser source that can be used to replace the pump line in previously proposed laser schemes and allow researchers to study the physics and feasibility of resonantly photo-pumped laser schemes. In this paper we use the X-FEL at 1174 eV to photo-pump the singly excited 1s2p state of Helike Ne to the doubly excited 2p3p state and model gain on the 2p3p-2p2s transition at 175 eV and the 2p3p-1s3p transition at 1017 eV. One motivation for studying this scheme is to explore possible quenching of the gain due to strong non-linear coupling effects from the intense X-FEL beam. We compare this scheme with photo-pumping the He-like Ne ground state to the 1s3p singly excited state followed by lasing on the 3p-2s and 3d-2p transitions at 158 and 151 eV. Experiments are being planned at LCLS to study these laser processes and coherent quantum effects.

We report spatial and spectral characterization an optical-field-ionized high-order harmonic-seeded soft-x-ray laser. We show that it can be controlled between a regular Gaussian shape and a Bessel profile exhibiting several rings via the IR laser pump intensity. The temporal coherence and spectral linewidth of both the seeded and unseeded soft-x-ray lasers were experimentally measured using a varying path difference interferometer. It showed that the high-order harmonic is subject to a strong spectral narrowing during its propagation in the plasma amplifier without rebroadening at saturation. Also, we present a new method to generate ultra-short x-ray laser pulses by using the laser-driven betatron source to photo-pump inner-shell transitions.

We describe our recent progress in the investigation of the spectral properties of collisional XUV lasers, including both experimental measurements and numerical calculations. Using a wavefront-division, variable path-difference interferometer, we have characterized the temporal coherence and the spectral width of an injection-seeded transient XUV laser emitted at 18.9 nm from a Ni-like Mo plasma. Our results show that the temporal coherence of the beam is significantly increased by the injection-seeded operation, compared to the standard ASE mode, in agreement with detailed numerical simulations. Using the PPP code we have calculated the intrinsic linewidth of the same lasing line over a range of electron density and at temperatures that are relevant to transient collisional pumping. We discuss the relative contributions of homogeneous and inhomogeneous broadening to the overall profile.

Seeding plasma-based soft-x-ray lasers (PBSXR) with high order harmonics (HOH) is a promising way to obtain fully coherent, short (hundreds of femtoseconds), tens of microJoules pulses. Nevertheless, up to date only 1 μJ, 1 ps pulses have been demonstrated seeding plasmas created from gas targets and solid targets. As the amplification process couples plasma hydrodynamics, atomic processes and the propagation of electromagnetic fields, a careful optimization of seed and amplifier properties is essential to reach multi-microJoule, hundreds of fs regime. Recent papers showed that short and wide (up to 1 mm) plasmas present an optimal gain zone and up to 20 μJ could be extracted when seeding. Nevertheless, the temporal duration and profile of the output beam is still not optimal. Simulations show that the HOH is weakly amplified whereas most of the energy is within a long (several picoseconds) wake induced by the HOH. In addition to this, these simulation pointed out the presence of deleterious Amplified Spontaneous Emission (ASE). In order to obtain intense pulses useful for practical applications is crucial to reduce the duration to hundreds of fs and obtain ASE-suppressed, structure-free (ideally only an amplified HOH) pulses. Using the 1D Bloch-Maxwell code DeepOne we will show that fully coherent, wake and ASE-suppressed, 15 μJ, 120 fs pulse can be obtained when optimizing at the same time both the seed and the plasma conditions.

Using the x-ray lasers as amplifiers of ultrashort x-ray pulses has been investigated as a scheme for ultimate light sources. For successful implementation, the characteristics of the scheme should be taken into account in designing the amplifiers. In this paper, the basic physics and characteristics of the scheme were analyzed by using the Maxwell-Bloch equations incorporating time-dependent gain, random spontaneous emission, atomic level degeneracy, and radiation polarization. The variation of the pulse parameters such as energy, bandwidth, pulsewidth, and polarization were explained based on a simple mechanism of pulse growth and also compared to that in the conventional x-ray lasers. These results should be the basic information for a practical implementation of the scheme.

The dependence of the yield of high-order harmonic generation (HHG) on several important experimental parameters has been successfully modeled in the last 20 years by taking into account the single atom response and propagation effects. We extended this description by adding a stimulated emission process and named it x-ray parametric amplification (XPA). Beyond the super-quadratic increase of the XUV signal, which can be explained only in a limited pressure range by HHG theory, other observed characteristics like exponential growth, gain narrowing, strong blue-shift, beam divergence, etc. and their dependence on laser intensity and gas pressure can be explained accurately only by the new XPA model. We experimentally demonstrated XPA in Argon in the spectral range of 40-50 eV in excellent agreement with the theory. XPA holds the promise to realize a new class of bright x-ray sources for spectroscopy.

We present experimental results, theory, and simulations demonstrating two novel sources of coherent X-ray radiation generated in the relativistic laser (>10¹⁸W/cm²) interaction with easily accessible, repetitive, and debris-free gas jet targets. The first source is based on a relativistic mirror reflecting a counter-propagating laser pulse. A strongly nonlinear breaking wake wave driven by an intense laser pulse can act as a semi-transparent relativistic flying mirror. Such a mirror directly converts counter-propagating laser light into a high-frequency (XUV or X-ray) ultrashort pulse due to the double Doppler effect. In the experimental demonstration with the 9 TW J-KAREN laser, the flying mirror generated in a He gas jet partially reflected a 1 TW pulse, providing up to ~10¹⁰ photons, 60 nJ (~10¹² photons/sr) in the XUV range (12.8-22 nm). The second source is demonstrated with the laser power ranging from 9 to 170 TW in experiments with the J-KAREN and Astra Gemini lasers. The odd and even order harmonics generated by linearly as well as circularly polarized pulses are emitted forward out of the gas jet. The 120 TW laser pulses produce harmonics with ~3×10¹³photons/sr (~600 μJ/sr) in the 120±5 eV spectral range. The observed harmonics cannot be explained by previously known mechanisms (atomic harmonics, betatron radiation, nonlinear Thomson scattering, etc.). We introduce a novel mechanism of harmonic generation based on the relativistic laser-plasma phenomena (self-focusing, cavity evacuation, bow wave generation), mathematical catastrophe theory which explains the formation of structurally stable electron density singularities, spikes, and collective radiation of a compact charge driven by a relativistic laser.

Circularly polarized high order harmonics in the extreme ultraviolet range (18 - 27 nm) have been obtained by a two steps process. Harmonics were generated from a linearly polarized infrared laser (40 fs, 0.25 TW) focused into a neon filled gas cell. The harmonics have then been circularly polarized by a four-reflector phase-shifter. The polarization of the harmonics have been measured using a rotating multilayer broadband mirror set at an incidence angle of 45°. Fully circularly polarized radiation has been obtained with an efficiency of a few percents. This is significantly more efficient than currently demonstrated direct generation of elliptically polarized harmonics. This demonstration opens up new experimental capabilities based on high order harmonics, for example, in materials science for time-resolved nanometric magnetic imaging.

LASERIX is a high-power laser facility leading to High-repetition-rate XUV laser pumped by Titanium:Sapphire laser. The aim of this laser facility is to offer Soft XRLs in the 30-7 nm range and auxiliary IR beam that could also be used to produce synchronized XUV sources. This experimental configuration highly enhances the scientific opportunities of the facility, giving thus the opportunity to realize both X-ray laser experiments and more generally pump/probe experiments, mixing IR and XUV sources. In this contribution, the main results concerning both the development of XUV sources(X-Ray lasers and HHG sources) and their use for applications are presented.

Heating of high energy density plasmas using extreme ultra-violet (EUV) and x-ray lasers is examined. Our modeling studies show that solid carbon and iron can be heated by focused X-ray laser pulses of irradiance 10¹⁷ Wcm^-2, duration 100 fs so that after a picosecond or so equilibration, LTE plasmas of temperatures up to 400 eV are produced in a uniform solid density of thickness close to one micron. Solid target heating experiments can also be carried out with laboratory based EUV lasers, but the temperatures achieved are < 20 eV. The equilibrium temperature reached with EUV and X-ray laser heating is strongly dependent on the photon energy, while the focused irradiance determines the thickness of heated material.

We have developed a soft x-ray laser (SXRL) interferometer capable of the single-shot imaging of nano-scaled structure dynamics. The interferometer consisted of the reflection optics including double Lloyd's mirrors and focusing optics, and the interference fringes are produced on the detector surface. The depth and lateral resolutions of the interferometer were about 1 nm and 1.8 μm, respectively. By using this interferometer, the initial stage (~50 ps) of the ablation process of the Pt surface pumped by a 70 fs Ti:Sapphire laser pulse was observed. The expansion speed of the surface estimated from the result (34 nm/50 ps) indicated that the nano-bubble structures were formed in the initial stage of the ablation. In order to observe the detailed dynamics, the temporal synchronization between the pump and probe pulses was improved to be 3 ps by adopting a portion of the SXRL and pump beams as the time fiducials, to which the pump and probe timing was adjusted by using the x-ray streak camera.

We present in this paper some effort and prospects in order to turn a high intensity infrared laser facility into a installation providing external users with EUV coherent sources. Development of an harmonic beamline based on quasiphase matching in capillary is presented. Plasma based soft x-ray laser scheme involving a plasma seeder and an amplifier is revisited with the aim of obtaining a robust fully coherent x-ray laser.

We discuss recent advances in the development of high repetition rate table-soft soft x-ray lasers resulting from research conducted at Colorado State University. Advancing saturated table-top lasers to shorter wavelengths we report the operation of gain-saturated sub-10 nm table-top lasers at 1 Hz repetition rate. We also present experimental results that show that injection-seeding of solid-target soft x-ray plasma amplifiers reduces the far field divergence by an order of magnitude and to allow for control of the far-field beam characteristics by tailoring the divergence of the seed. We finally discuss progress towards the development of high repetition rate compact all-diode-pumped soft x-ray lasers. We have operated the front end of the diode-pumped soft-ray laser driver at 100Hz repetition rate, obtaining sub-5 ps optical laser pulses of 100 mJ energy.

We present an experimental design to independently pump two soft X-ray laser media suitable for a seed-amplifier configuration. Both the seed and the amplifier target are operated in the TCE scheme utilizing the DGRIP technique with its intrinsic travelling wave excitation. Controlled injection of the seed X-ray laser into the amplifier medium is realized via a spherical XUV mirror. The experimental design is perfectly appropriate for benchmarking combined simulations of the ARWEN and DeepOne code. A first experiment at the PHELIX laser utilizing this scheme has been conducted, demonstrating signs of amplification and allowing for the direct measurement of the gain life time of a Ni-like silver SXRL.

By seeding amplifying plasmas pumped with the so-called Transient collisionnal excitation scheme, the amplified pulse seems to be limited to an energy of several 10's of μJ. Aiming to attain several mJ, we study the seeding of plasma pumped by long laser pulse. Thanks to our time-dependant Maxwell-Bloch code, we demonstrate that direct seeding with femtosecond pulse is inefficient. We also study the amplification of pulse train with the drawback of re-synchronizing the pulses. We proposed and studied the amplification of high harmonic seed stretched by a grating pair, amplified finally compressed. We consider off-axis diffraction on the gratings for maximizing their efficiency. Considering the phase deformation induced by the amplification and the spectral narrowing the final pulse is 230 fs in duration and 5 mJ.

We report a recent experiment where the first hard x-ray beam line, X-ray Pump Probe (XPP) instrument using the SLAC National Accelerator Laboratory's Linac Coherent Light Source (LCLS) free electron laser, was used to heat thin foils to high energy densities ~ 10⁷ J/cm³. An intense 9 keV, 60 fs (FWHM) duration beam with energy of 2 - 4 mJ at the XPP beam line was focused using beryllium lenses to an irradiance approaching 10¹⁶ Wcm^-2. Targets of 0.5 - 3.5 μm thick foils of Ag and Cu were studied using a suite of diagnostics including Fourier Domain Interferometry, energy calorimetry and grating and crystal spectrometers. The experimental details and spectroscopic results from the campaign will be described. Preliminary results indicate that the target is heated relatively uniformly to a temperature lower than 20 eV.

An inline diagnostics device was developed to measure the intrinsic shot-to-shot intensity and position fluctuations of the SASE-based LCLS hard X-ray FEL source. The device is based on the detection of back-scattered X-rays from a partially-transmissive thin target using a quadrant X-ray diode array. This intensity and position monitor was tested for the first time with FEL X-rays on the XPP instrument of the LCLS. Performance analyses showed that the relative precision for intensity measurements approached 0.1% and the position sensitivity was better than 5 μm, limited only by the Poisson statistics of the X-rays collected in a single shot.

We present simulations that allow studying Auger heating and the subsequent evolution of the radiation emission of near solid density matter. Particular emphasize is paid to the multi-charge state inverse Auger-effect in dense plasmas which is proposed to explain the target emission when the conduction band at solid density becomes more atomic like as energy is transferred from the electrons to the ions. Simulations are discussed along with the first available experimental data.

Reduced replicas of a periodic transparent mask were printed in the surface of a photoresist using Talbot effect. The transparent mask (or Talbot mask) was composed of an array of unit tiles distributed in a square matrix. The mask was illuminated by a coherent table top soft X-ray laser. To achieve the demagnification effect, the illumination beam was reflected in a spherical mirror. At determined positions given by the Talbot distance reduced replicas of the mask were obtained. The Talbot images produced smaller copies of the mask in the surface of a photoresist. Calculations based on Kirchhoff-Fresnel theory shows a good agreement with the experimental results. The limitations for this method are further discussed.

The theory of reflection image formation at coherent slant illumination is specified for the case of confined objects. The approach is based on the parabolic wave equation with a boundary condition posed on an object surface that is tilted in respect to the beam k-vector. The result is obtained in the form of a new propagation integral. 3D simulation of coherent reflection imaging at grazing angles of illumination in a lens optical scheme is performed and discussed.

We present a method for the optimization of the illumination in soft x-ray (SXR) full-field microscopes. The method consists of imaging a single periodic grating with a period large compared to the wavelength of the illumination and obtaining its Fourier spectrum in two orthogonal directions. The analysis of the cut-off frequency along the two perpendicular directions allows the identification of angled illumination, which can be corrected in-situ by using the Fourier analysis iteratively. The ability to characterize the illumination conditions and aberrations in the EUV/SXR microscopes with a fast and simple analysis is critical to achieve the best quality images with the highest spatial resolution.

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^-1 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 (BaF₂, LaF₃) were used, either in the form of windows or coated onto glass slides.

We measured the linewidth of a λ= 46.9 nm neon-like argon capillary discharge soft x-ray laser. A wavefront division interferometer based on a pair of dihedrons was used to resolve the laser line measuring the variation in the interference fringe visibility for different optical path differences. We measured a relative linewidths of Δλ/λ = 3-4 10^-5. No significant re-broadening was observed when the length of the laser medium was increased beyond the saturation length due to effects that homogenizes the line profile.

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.

The new scheme is presented for Ne-like Ti X-ray laser driven by a single laser pulse. According to the self-similarity method, we analyze the properties of Ne-like Ti slab plasma generated by irradiation of femtosecond laser with various front edges. Scaling laws for the temperature, scale length, and electron density are obtained. The characteristics of scaling laws at different input parameters are analyzed. The results show that X-ray laser can be generated by a single femtosecond laser. During the period of pulse front edge, the characteristics of temperature, scale length, and electron density are affected by the growth tendency of light intensity. The femtosecond laser with a gentle front edge is profitable to drive X-ray laser. Our results provide a new program for experiments using a single laser pulse to drive X-ray laser.

We have observed guiding of 10¹⁶-W/cm² ultrashort laser pulses in a discharge-produced plasma channel in an argon-gas-filled 1-cm long alumina (Al₂O₃) capillary. An optimum timing of the laser pulse injection was around 150 ns for the discharge with a peak current of 200 A at the discharge time 150 ns. One-dimensional magnetrohydrodynamic (MHD) code was used to evaluate the average degree of ionization of Ar in the preformed plasma channel. In the preformed plasma the averaged ion charge state of Ar³⁺ was obtained from the observed visible emission spectra and from the MHD simulation. The spectrum of the laser pulse in the preformed by the discharge argon plasma channel was not changed and was well reproduced in the particle-in-cell (PIC) simulation.

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.

For testing and application purposes we have built a new small Marx generator capable to run in a repetitive regime. Its repeating frequency is currently up to 1 Hz. The generator is covered by metal sheets and feeds the CAPEX facility and ensures its full independence on the CAPEX-U machine (using another Marx generator). This paper reports on the first experimental results of a new experimental set-up of the CAPEX apparatus (repetitive lasing at 46.9 nm), mainly on set-up description, electrical parameters, and laser pulse stability in the repetitive regime.

XUV radiation from nitrogen filled capillary discharge plasma was diagnosed using a 10⁴ grooves/mm SiNx free-standing transmission grating. The resolution bandwidth of 0.3 nm was achieved. Time dependence of 13.4 nm line emitted power was recorded by photomultiplier in order to verify inherence of resonant radiation emission corresponding to NVII 2-3 laser transition. An increase of emitted power is expected during the pinch decay caused by recombination processes. We report here results obtained with 90 mm long capillary discharge supplied by a current pulse with maximum amplitude of 50kA and quarter-period of 80 ns. This high-current pulse was generated by a 1.5 ohm water line high-voltage generator which is used for underwater wire explosion experiments and which was adjusted for capillary discharge design using results of PSPICE simulations. Initial nitrogen pressures were varied in the range of 20 ÷ 500 Pa. MHD and kinetic simulations of the discharge plasma were performed and compared with experimental data. Simulations were performed with presumption of wall ablation. The capillary wall and electrodes material emission lines were also identified in measured spectra.

To understand the origin of the non-uniform spatial structure of the soft x-ray emission in the laser produced plasmas, a possibility for plasmas to have phase transition processes is investigated. Phase diagram of the plasma is investigated through an analysis of the radiation pressure and its dependence on temperature and density. A computational method based on a particle model to reproduce the structure formation is also presented.

We report recent experimental measurements of the duration of a Ni-like Mo transient XUV laser emitted at 18.9 nm and generated under GRIP geometry at the LASERIX facility. These measurements have been performed using an ultra-fast X-ray streak camera (AXIS Photonique). A new trigger line was implemented, yielding a shot-to-shot jitter of less than 10 ps. The KBr photocathode of the streak camera was positioned close to the plane of the magnified near-field image of the XUV laser emitting aperture. Saturation in the streak camera was avoided by carefully adjusting the signal level of the focused image. The sweep speed was calibrated in-situ by generating a double-pulse XUV laser. Finally the temporal resolution of the instrument was measured directly with a femtosecond, high-order harmonic pulse. XUV laser pulse durations as short as 2.8 ps were measured for a 20° GRIP angle and a 0.75 ps GRIP pulse duration. We present and discuss our study of the effect of these two parameters on the measured pulse duration.

The atomic structure and x-ray pumping of neonlike Fe xvii have been calculated and modeled under free-electron laser excitation conditions using the Flexible Atomic Code. Specifically, pumping of the (2p_3/23s_1/2)_2,1, (2p_1/23s_1/2)₁, (2p_3/23d_5/2)₁, and (2p_1/23d_3/2)₁ levels that connect with the ground state (2s²2p⁶)₀ by the so-called M2, 3G, 3F, 3D, and 3C transitions have been studied. In addition, the spectrum of sodiumlike Fe xvi has been modeled to account for possible line coincidences with the neonlike spectrum. The calculations include oscillator strengths, radiative transition probability rates, autoionization rates, non-resonant photoionization cross sections, and line emissivities.

We present a comprehensive theoretical analysis of the conditions that lead to absorption limited generation of high order harmonics in the case of low energy, long pulse duration infrared pump laser beams. With these pump laser beam parameters, tight focussing is necessary to reach the high laser intensity required for high non linear response at the atomic level. A limitation of phase-matching is then induced through the so-called Gouy phase shift. We explain how the positive atomic dispersion can compensate for that effect and derive the conditions for perfect phase matching. The influence of gas pressure, harmonic order, maximum laser intensity and focussing conditions are analysed. This work enables a precise separation between pure classical harmonic from possible XPA effects discussed in a recent experiment. It also constitutes a benchmarking of a code under development for quasi phase matching scheme investigation.