This PDF file contains the front matter associated with SPIE Proceedings Volume 8849, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

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^-1 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.

The results of development and applications of the secondary sources at PALS Center will be presented. Currently the iodine system and the Ti:Sapphire system are operating at the PALS Center as driving lasers for generation of secondary sources. The iodine system with net energy of 1kJ is used for QSS X-ray lasing schemes. The most robust and most energetic QSS scheme with this driver is the Ne-like Zn X-ray laser, which is working here as standard user beamline for diverse applications. Recent experiment devoted to temporal coherence measurement shows possibility to amplify short duration X-rays. The second system with high rep rate is Ti: Sapphire laser chain with peak power 20TW. This laser system is used for generation high order harmonics and transient collisionally excited X-ray lasers.

For quasi-phase matching of high-harmonic generation at short wavelengths, a beat-wave pulse train with 66-fs pulse separation is generated from a two-color Ti:sapphire amplifier system. By using such pulse train to collide with the main driving pulse for harmonic generation in the interacting media, quasi-phase matching can be achieved. The 66-fs pulse separation matches to a quasi-phase-matching zone length of 4.9 μm, which corresponds to a dephasing length for 3-nm harmonic generation under 1.0×10¹⁸ cm⁻³ plasma density. The pulse train energy is 100 mJ, sufficient to support more than 1000 quasi-phase-matching zones.

Metallic sodium (Na) was proposed as a transparent material in the vacuum ultra-violet (VUV) spectral range in 1930s and in 1960s. However no clear transmission has ever been demonstrated. In this paper we describe firstly the direct measurement of actual transmittance of a sodium samples in a spectral range longer than 115 nm which corresponds to the shortest transmission wavelength of magnesium fluoride (MgF₂) windows, resulting in several tens of % transmittance of a 3 mm-thick solid sodium sample including MgF₂ windows at the wavelength of ~120 nm. We also find very weak temperature dependency of the transmittance up to 150 degrees centigrade where the solid sample is melted at 97 degrees. The measured transmittance pushes us to make a simple imaging experiment illuminated by the VUV light through a 2-mm thick sodium sample, resulting in obtaining a clear image composed of 100 μm diameter tungsten mesh recorded on a two dimensional Charge Coupled Device detector. The result also opens a way to construct an optical imaging device for objects inside or through a solid or a liquid sodium medium. According to the present experiment, we can make a continuous real time transmission imaging for a liquid sodium sample if we use proper optical setup including an intense continuous VUV source or high repetition rated intense coherent source for holographic data acquisition. Such an experiment opens up a way to perform transmission imaging through or inside a sodium medium for characterization of hydrodynamic and material properties.

After the successful demonstration of the hard X-ray self-seeding at LCLS, an effort to build a system for working in the soft X-ray region is ongoing. The idea for self-seeding in the soft X-ray region by using a grating monochromator was first proposed by Feldhauset. al. The concept places a grating monochromator in middle of the undulators and selects a narrow bandwidth “seed” from the SASE beam produced by the upstream section of undulators, which is then amplified to saturation in the downstream section of the undulators. The seeded FEL beam will have a narrower bandwidth approaching the transform limit. The challenge is to accommodate a monochromator and refocusing system as well as the electron beam magnetic chicane into a very limited space. The Soft X-raySelf Seeding system replaces only a single undulator section of ~ 4 m. Theoverall project and the expected FEL performances are described elsewhere. Here we present the detailed optical design solution, consisting of a fixed incidence angle toroidal blazed grating with variable groove density, a rotating plane mirror (the only required motion for tuning the energy) to redirect the selected monochromatic beam onto an exit slit, and two more mirrors, one sphere and one flat, to focus and overlap the ‘seed’ onto the electron beam in the downstream undulators.

X-ray free-electron lasers (FELs) open up new frontiers in photon science, which is a consequence of their outstanding characteristics in terms of the generated FEL radiation. The laser-like x-ray radiation with high spectral brightness, wide tunability, and almost full spatial coherence meets many requirements of experimental techniques in photon science. Femtosecond radiation pulses extend the capabilities of these unique accelerator-based light sources. However, the ability to temporally characterize x-ray pulses from FELs will underpin their exploitation in experiments ranging from single-molecule imaging to extreme timescale x-ray science. This issue is especially acute when confronted with the characteristics of current generation x-ray FELs, as most parameters uctuate strongly from pulse to pulse. Here, we review recent advances in the temporal characterization of x-ray pulses at FELs, with the emphasis on techniques with femtosecond resolution and single-shot capability. Various measurements in the soft and hard x-ray regime of current generation x-ray FELs are presented and discussed.

We have proposed and developed a new hybrid microscopy system using a soft x-ray microscope and a fluorescence microscope imaging the same biological cells at the nearly same time. Combining the powerful advantages such as high spatial resolution of the soft x-ray microscope and the accurate organelle identification of the fluorescence microscope, we can observe fine structures of the cellular organelles in live hydrated biological cells in situ. Staining the cells with several fluorescent dyes such as Mito-tracker, Phalloidin, and DAPI, the soft x-ray images of the cells have been directly compared with the fluorescent images and the cellular organelles such as mitochondria, actin filaments, and chromosomes in the soft x-ray images have been clearly identified. Since the soft x-ray microscope has higher spatial resolution than that of the fluorescence microscope, not only shape of the cellular organelles but also the fine structures of the cellular organelles of the live biological cells have been clearly observed for the first time.

Mg/SiC multilayer coatings with Al-Mg corrosion barrier layers and with high reflectance in up to three narrow wavelength bands within the 25-80 nm wavelength region have been developed for EUV laser source applications. The physics of the spontaneous intermixing and amorphization of the Al-Mg corrosion barriers for Mg/SiC multilayers are currently being investigated in detail. Initial results from studies of the long-term reflectance properties of corrosionresistant Mg/SiC multlayers are also discussed.

We have developed the pump and probe soft x-ray imaging technique of the metal surfaces during the femtosecond laser ablation by using the laser-driven soft x-ray laser at the wavelength of 13.9 nm. The pumping laser used for the ablation was a Ti:Sapphire laser pulse with the duration of 80 fs pulse at a central wavelength of 795 nm, and had a Gaussian spatial profile. By using the x-ray interferometer, the time resolved image of nano-scaled ablation dynamics of the platinum was obtained. At the timing of 36 ps after the femtosecond laser irradiation, the maximum surface expansion and expansion speed were measured to be about 60 nm and 1,700 m/s, respectively. We have compared between the experimental result of the surface expansion and the profile of the ablated hole measured by the atomic force microscopy, and discussed the fluence dependence of the femtosecond laser ablation. These results lead to better understanding of the initial process of the laser ablation dynamics.

To study the interactions between a soft x-ray laser (SXRL) and various materials, we irradiated Al, Au, Cu, and Si with the SXRL beam pulses having a wavelength of 13.9 nm and duration of 7 ps. Following the irradiation, the induced structures were observed using a scanning electron microscope and an atomic force microscope. With single pulse irradiation, conical structures were observed on the Al surface, and ripple-like structures were formed on the Au and Cu surfaces. The conical structures were destroyed under multiple SXRL pulse irradiation. On the other hand, the developments of modified structures were observed after multiple pulse irradiations on the Au and Cu surfaces. On the Si surface, deep holes, that seemed to be molten structures induced by the accumulation of multiple pulse irradiations, were found. Therefore, it is concluded that the SXRL pulse irradiations of various material surfaces cause different types of surface modifications, and the changes in the surface behaviors are attributed to the differences in the elemental properties of each materials, such as the melting point and the attenuation length of x-rays.

Mass spectrometry plays a vital role in the direct examination of the chemical composition of solids. We have introduced the use of soft x-ray laser ablation for mass spectrometry imaging. Here we demonstrate the method potential for composition depth profiling of multilayer stacks consisting of tens of nanometers thick metal and dielectric films.

Ultrafast X-ray absorption spectroscopy (UXAS) offers the opportunity to investigate function-structure relationships of complex organic molecules or biological functional subunits without the need of crystallization. Of special interest from the viewpoint of structural biology is the region of K-edges of transition metals between 5 and 10 keV. Regardless of successful application of time-resolved diffraction techniques to investigations of crystal dynamics using synchrotron and laboratory based sources there are only very few examples for application of UXAS to revealing the structural dynamics in biomolecular systems. This is mainly caused by the lack of broadband ultrafast x-ray sources as well as of appropriate optics adapted to these sources. Due to the long-data-recording time in UXAS experiments the sample integrity is mainly determined by the average power of the pump pulses inducing the structural changes. Using a fixed energy of the pump pulse the latter one is determined by the repetition rate of the pump laser. In this paper we discuss the prospects of UXAS comparing fs laser plasma sources with different repetition rates in combination with tailor-made optics based on highly annealed pyrolytic graphite (HAPG).

Here we present a next-generation experimental setup for high-resolution X-ray spectroscopy of solid and liquid samples in the soft X-ray region to elucidate the complex molecular structures of (bio)chemical systems. The setup consists of a main target chamber, a target holder for either solid samples or a liquid jet delivery system, and a high-resolution soft X-ray grating spectrometer. This setup is in commissioning at PETRA III, presently one of the most brilliant storage ring based X-ray radiation sources in the world. The newly designed grazing incidence grating spectrometer is utilized for high-resolution measurement in the XUV range from 1 nm up to 6 nm.

Ultra-intense X-ray sources have opened new avenues by creating new states of matter or probing and imaging living or inert matter. Free-electron lasers have a strong leadership by delivering pulses combining femtosecond duration and 10s of microJoules to milliJoule energy. However, these sources remain highly expensive limiting their number to a few worldwide. In parallel, laser-pumped soft X-ray lasers hold outstanding promises having demonstrated the most energetic monochromatic soft x-ray pulse and being intrinsically fully synchronized with any secondary source of the pump laser. Since the first successful demonstration of amplification of a high harmonic pulse in a plasma from gas in 2003 and from solid in 2008, we have developed an extensive numerical study. 2D hydrodynamic simulations showed that optimized Transient Collisional Excitation plasma amplifiers, may store up to 0.4 mJ in the population inversion. If carefully seeded, pulses of 80 fs and 20 μJ might be generated with table-top lasers (10J). As the energy extracted is far from the milliJoule requirements of most exciting applications, we studied the seminal experiment of Ditmire et al who seeded a plasma emitting milliJoules in the form of Amplified Spontaneous Emission (ASE).We retrieved and explained for the first time the experimental result (ASE 1,000 times stronger than amplified seed). We thus proposed and fully modeled the transposition of the so-called Chirped Pulse Amplification (CPA) in the soft X-ray range, showing that 6 mJ, 200 fs, fully coherent soft X-ray pulse is accessible with compact pump lasers.

One promising way to achieve ultra-short pulse amplification in XUV laser amplifiers rests on nonlinear processes involving such phenomena as Rabi oscillations. A criterion for such behavior to occur in currently available XUV lasers is derived. Two-dimensionalMaxwell-Bloch simulations of short pulse amplification are presented in the case of Ni-like Ag pumped in grazing incidence and seeded by a high-order harmonics pulse. From numerical seed parameter optimization, it is shown that an ultrashort, high intensity, quasi-π XUV pulse can be produced in that system.

An experiment was set up to measure the wavefront of an injection-seeded soft x-ray laser based on a solid-target plasma amplifier. The 43rd harmonic signal from a Ti:Sa laser was used to seed a molybdenum plasma amplifier operating in the λ=18.9 nm line of Ni-like Mo. A Hartmann wavefront senor with an accuracy of λ/32 rms at this wavelength was employed to measure the wavefront of both the high harmonics seed and the seeded soft x-ray lasers. A significant improvement in wavefront aberration from 0.51±0.04λ rms to 0.25±0.03λ rms was observed as a function of plasma column length. The variation of wavefront characteristic by the time delay between the injection of the seed and the peak of soft x-ray amplifier pump was studied in this paper.

The table-top generation of high average power coherent soft x-ray radiation in a compact set up is of high interest for numerous applications. We have demonstrated the generation of bright soft x-ray laser pulses at 100 Hz repetition rate with record-high average power from compact plasma amplifiers excited by an ultrafast diode-pumped solid state laser. Results of compact λ=18.9nm Ni-like Mo and λ=13.9nm Ni-like Ag lasers operating at 100 Hz repetition rate are discussed.

FERMI@Elettra is the first seeded VUV/soft X-ray FEL source. It is composed of two undulatory chains: the low energy branch (FELl) covering the wavelength range from 20 nm up to 100 nm, and the high energy branch (FEL2, employing a double stage cascade), covering the wavelength range from 4 nm up to 20 nm. At the end of 2012 FELl has been opened to external users while FEL2 has been turned on for the first time having demonstrated that a double cascade scheme is suitable for generating high intensity coherent FEL radiation. In this paper we will share our experience and will show our most recent results for both FERMI FELl and FEL2 sources. We will also present a brand new machine scheme that allows to perform two-colour pump and probe experiments as well as the first experimental results.

The number of proposals for LCLS science has rapidly increased as all six LCLS x-ray instruments have come online. It created rising demand on beam time. Statistics shows that only about 25 % of LCLS proposals can be allocated beam time. One way to increase access is to share the x-ray beam between the different instruments. The purpose of this study is to quickly switch the x-ray beam between the Matter in Extreme Conditions (MEC) Instrument and the Coherent X-ray Imaging (CXI) or X-ray Correlation Spectroscopy (XCS) Instruments, in order that two of the instruments can perform experiments simultaneously. In the most common operational mode, the MEC Instrument uses one x-ray pulse every 10 minutes, limited by the repetition rate of the high power nanosecond laser system. The MEC M3H mirror steers the x-ray beam to the MEC Instrument from the XCS or CXI Instruments. If the M3H mirror could switch the x-ray beam to MEC within a fraction of the 10 minutes waiting time, multiplexing of the x-ray beam would be achieved. The M3H mirror system has two motion stages for translation and rotation. The long path, 230 m, from the mirror to MEC hutch makes the pointing resolution 0f 100 microns and stability requirements challenging. The present study investigates such capabilities by measuring the correlation between the translation speed and the beam pointing reproducibility. We show that mirror translation can multiplex the LCLS x-ray beam.

During the last years, scanning coherent x-ray microscopy, also called ptychography, has revolutionized nanobeamcharacterization at third generation x-ray sources. The method yields the complete information on the complex valued, nanofocused wave field with high spatial resolution. In an experiment carried out at the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS) we successfully applied the method to an attenuated nanofocused XFEL beam with a size of 180(h) × 150(v)nm² (FWHM) in horizontal (h) and vertical direction (v), respectively. It was created by a set of 20 beryllium compound refractive lenses (Be-CRLs). By using a fast detector (CSPAD) to record the diffraction patterns and a fast implementation of the phase retrieval code running on a graphics processing unit (GPU), the applicability of the method as a real-time XFEL nanobeam diagnostic is highlighted.

A PW Ti:Sapphire laser with 30-J energy and 30-fs pulse duration has been developed at GIST and applied to generate x-rays and energetic charged particles. We present the status and plan of developing ultrashort x-ray sources and their applications. We successfully demonstrated x-ray lasers and their applications to high-resolution imaging. In addition, we plan to generate high flux x-ray/gamma-ray sources using the PW laser.

We describe a table top nanofabrication system that utilizes the coherent Talbot imaging effect and a table-top soft x ray laser to implement a defect-free compact nanolithography tool. The Talbot lithography provides a robust and simple setup capable of printing periodic structures over millimeter square areas free of defects. Test structures were fabricated into metal layers showing a complete coherent extreme ultraviolet lithographic process in a table-top system.

Using a zone plate interferometer we have demonstrated image-plane holographic microscopy employing the radiation of a compact capillary discharge Ar laser operating at 46.9 nm for illumination. In this paper we describe a full analysis of the spatial resolution of the system and demands on the coherence of the radiation. The analysis shows that the resolution in the amplitude and phase images is mainly determined by the numerical aperture of the imaging element and the wavelength of the illumination. The resolution, however, is not affected by the degree of coherence, which only reduces the field of view. We also show rather low demands on temporal coherence due to the common-path interferometric geometry.

We present a coherent diffraction imaging (CDI) experiment using a high-frequency discharge plasma based, extreme ultraviolet (EUV) source. By using different illumination geometries, we generated EUV beams witha varying degree of spatial coherence, which were used to produce far field diffraction patterns from test objects. We then successfully reconstructed an illumination wavefront defined by a circular aperture. The present work explores the feasibility of compact tabletop CDI using a discharge plasma EUV source emerged from the technology development of EUV lithography, which can potentially find application in nanoscience and metrology.

We present in this paper a laser-driven coherent EUV beamline resulting from the combination of a versatile high-order laser harmonic generator with a robust plasma-based EUV laser amplifier. Both devices can be used separetely or in synergy. Seeding of the plasma amplifier by a high-order harmonic beam leads to a strong improvement of the EUV laser beam divergence and uniformity. Moreover the system can be turned easily into a IR pump-XUV probe setup for plasma opacity probing. The possibility to generate two separate harmonic sources from the same gas cell offers the opportunity to explore EUV pump-EUV probe experiments.

The potential for coherent extreme ultra-violet (EUV) light in probing laser-produced plasmas is investigated. New results are presented to demonstrate that EUV radiation can be employed to measure heat penetration into solid targets from electrons using the signature of a change of opacity due to heating. We examine, in particular, the effects of hot electron heating of targets. In addition, phase variations after transmission through a laser-irradiated target change the subsequent propagation of the radiation, suggesting a simple diagnostic measuring the far-field footprint of coherent EUV radiation can be a useful measurement of the uniformity of target heating.

A comprehensive simulation study is presented, examining the interaction of an EUV capillary discharge laser, operating at 46.9nm, within carbon at solid density. By incorporating a detailed model of photoionization, equation of state calculations, electronic term accounting and refractive index calculation into a pre-existing 2D radiative-hydrodynamic code POLLUX, target ablation and subsequent plasma expansion has been simulated for target material under intense (10¹¹ W cm^-2) EUV irradiation. Unique ablation based on direct photoionization by EUV photons creates solid density plasma with a temperature below 20eV. Plasma in this warm dense matter state is of particular interest to inertial con_nement fusion research. A reduction in focal spot size, due to a decrease in the di_raction limit, combined with increased target penetration allows for high-aspect ratio hole drilling and a signi_cant increase in the ejected target mass. This work outlines a comprehensive computational environment used to simulate the EUV/x-ray laser interaction within solid material and expanding plasma.

We have developed the standalone, coherent scatterometry microscope (CSM) for the inspection of extreme ultraviolet (EUV) lithography mask. The low divergence, coherent high-order harmonic (HH) was generated as coherent light source for CSM at a wavelength of 13.5 nm using a commercial laser system. The HH enable us to obtain the high contrast diffraction image from the mask. The diffraction light from the 2-nm wide line-defect and tens-nm size point-defects in the mask has been observed successfully with the system.

X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. Most experiments are currently done at large laser facilities that can create bright X-ray sources, however the advent of the X-ray free electron laser (XFEL) provides a new bright source to use in these experiments.One challenge with X-ray Thomson scattering experiments is understanding how to model the scattering for partially ionized plasmas in order to include the contributions of the bound electrons in the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and the electron-electron plasmon scattering and add the contribution of the bound electrons in the partially ionized plasmas. We validated our model by analyzing existing beryllium experimental data. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. We also show examples of experiments in CH and Al that have bound contributions that change the shape of the scattered spectra.

This paper reports on the experimental results of CAPEX apparatus (repetitive discharge-pumped laser working at the wavelength 46.9 nm), mainly on its beam characteristics (laser pulse energy, and laser beam profile). The divergence of XUV laser for two capillary lengths (232mm/400mm) is presented as well.

Short-period laser ablation of a platinum solid target was investigated through three-dimensional classical molecular dynamics simulations using the embedded atom method potential. The platinum target was ablated by an ultrashort-pulse laser with three different fluences near the ablation threshold and single 100-fs pulse. Although each laser fluence causes melting and evaporation of the target surface, ablation processes are morphologically different. When the laser fluence is just above the ablation threshold, the surface layer of the solid target breaks away, and so-called spallation occurs. With the moderate laser fluence, homogeneous nucleation of nano-sized clusters takes place in the liquidized layer at the surface, resulting in the homogenization in the emitted cluster size, while the surface layer fragments and vaporizes with the higher fluence. Moreover, in the spallation regime, the recreated surface has nano-sized roughness and is formed after the surface oscillates with a rv20-ns period. This inherent roughness formation may be a seed of the nano-sized regular structure observed by past experiments with repetitive pulses.