Soft X-ray Region Research Articles

Multicolor wavefront sensing using Talbot effect for high-order harmonic generation

High-order harmonic generation (HHG) using femtosecond lasers is an efficient and cost-effective method for producing attosecond and femtosecond light in the extreme ultraviolet and soft x-ray regions. This technique generates high-brightness harmonic beams with stable spectra, intensity, and wavefronts, making it a valuable tool for applications such as nanoimaging, material metrology, and studies of ultrafast dynamics. Accurate wavefront measurement of HHG beams is essential for analyzing complex incident fields, understanding atomic responses, and enhancing applications such as attosecond-driven ultrafast processes. However, quantifying the wavefronts of each harmonic component remains a challenge. Here, we present a multicolor wavefront sensing for HHG beams using the Talbot effect, validated theoretically and experimentally. Our method reconstructs individual high-order harmonic wavefronts with a spectral resolution of 4.8 eV, capturing multiple harmonic wavelengths in a single scan along the propagation direction, without the need for additional spectrometers or complex calibrations. This approach surpasses conventional Talbot effect wavefront sensing, which typically relies on quasimonochromatic waves or wavelength-averaging. Additionally, we introduce a high-accuracy centroid detection strategy that reduces tracking errors to the thousandths of a pixel while maintaining high computational efficiency, paving the way for broader applications in imaging and metrology. Published by the American Physical Society 2024

Investigating the consistency of the shape and flux of X-ray reflection spectra in the hard state with an accretion disk reaching close to the black hole

The observed spectra from black hole (BH) X-ray binaries (XRBs) typically consist of two primary components. A multitemperature blackbody originating from the accretion disk in the soft X-ray, and a power law-like component in the hard X-ray, due to the Comptonization of soft photons by the hot corona. The illumination of the disk by the corona gives rise to another key component known as reflection. A fraction of the incident hard X-ray radiation is naturally absorbed and re-emitted as a blackbody at lower energies and referred to as the ``reprocessed blackbody''. For densities relevant to XRBs and typical ionization values, the reprocessed blackbody may become significant in the soft X-ray region (approximately 0.1-1.0 keV) and should be noticeable in the observed spectra as a consequence of reflection. The absence of any blackbody component in the low/hard state of a BH XRB may not be consistent with the reflection of highly irradiating flux, observed as a power law from an appropriately dense disk of XRB. We focus on the low/hard state of the BH XRB MAXI J1820+070. In contrast to previous works, we simultaneously fit the shape and flux of the reflection spectra. This allowed us to estimate the correct density and ionization of the slab as well as the corresponding reprocessed blackbody. Our fitting of the representative observation of the BH XRB low/hard state suggests that the disk may, in principle, extend very close to the BH, even though the reprocessed thermal emission (due to disk illumination) remains cold (and thus low) enough to be consistent with the data in contrast to the results of a previous study. The inner reflection component is highly ionized and its fit is primarily driven by its contribution to the continuum, rather than by the shape of the relativistic iron line. The reprocessed blackbody cannot help determine whether the disk extends close to the BH or not in the hard state. For this specific observation, the flux in inner reflection component turns out to be quite low with respect to the outer reflection or power law. The outflowing slab corona covering the inner region of the disk could be the plausible geometry of the source, with the underlying disk approaching near to the BH.

Influence of the magnetic field on the emission of hot electrons and ions from ablative plasma produced from a disc-coil target irradiated by the 3rd harmonic of the PALS iodine laser

Abstract Experimental and theoretical study of plasma processes affected by strong laser generated magnetic fields is reported. The PALS laser system operating at 3ω (438.5 nm) delivered intensities up to 1×1016 W/cm2 on targets. By using a special target system consisting of a Cu foil connected to a sub-mm coil, magnetic fields up to the level of 10 T were generated. This is 1.4 times higher than the field generated with a 1.315 μm (1ω) laser beam at a comparable intensity. We found that these fields were sufficiently strong to modify plasma blow-off from the foil which resulted in the changed expansion dynamics and increased energy of hot electrons by 20-40% compared to the plasma unaffected by the magnetic field. To obtain complementary experimental data, a complex diagnostic system was used enabling the visualization of the plasma expansion process both in visible light (3-frame composite interferometry) and in the soft X-ray region (4-frame pinhole X-ray camera) together with measurements of the hot electron parameters using two-dimensional imaging of the Kα line emission from the Cu target and electron spectroscopy. Experimental data obtained from the angular distribution of electron energy spectra were used for three-dimensional (3D3V) numerical PIC simulations using a modified EPOCH code. By including interactions between ions, protons, hot and thermal electrons in forward and backward propagating particles, the effects of the magnetic field on the flux of hot electrons were visualized and compared with the experiment. The PIC simulation confirmed that the interaction of the hot electron flux with the magnetic field generated by the target-coil system leads to an increased flux energy. However, this increase is accompanied by increased complexity of the spatial structure and heterogeneity of the flux as well as its angular divergence.

Structural development on Ru and RuO2 electrodes during oxygen evolution – a soft X-ray absorption spectroscopy approach

Time resolved in-situ X-ray absorption spectroscopy (XAS) in soft X-ray region was used to characterize polarized interphase on Ru and Ru oxide based electrodes under oxygen evolution reaction (OER) conditions. XAS spectra were used to align the type and population of oxygen-containing species formed at electrodes at anodic potentials with local electronic structure of the OER catalyst. The soft XAS data do not identify a single rate limiting process for OER at potentials negative to 1.7 V vs. RHE. Individual intermediates of the oxygen evolution process coexist at the surface at potentials preceding the actual OER onset. The OER is accompanied with redistribution of the electron density resulting for a start of the catalytic cycle reflecting the transfer of the electrons from water to the external circuit during catalyzed water oxidation. The observed spectral behavior indicates a confinement of the OER to the coordination unsaturated sites (cus) at the surface.

Feasibility study of fabricating 20 nm resolution dielectric Fresnel zone plates with ultrahigh aspect ratio for EUV optics

EUV light optics are either reflective or diffractive due to the substantial absorption characteristics by almost all materials. Despite great successes in manufacturing integrated circuit chips, reflective EUV optics are still unfriendly to small-to-medium enterprise (SME) because of the enormous costs. Recently, diffractive EUV optics has come to the light in hopes to be able to establish manufacturing nanoscale products and inspecting nanoscale structures. Diffractive zone plates with high resolution in EUV wavelengths are urgently needed. This paper reports our latest success in developing 20 nm resolution zone plates for focusing and imaging in the EUV and soft X-ray regions. It firstly discusses the diffraction efficiency of FZPs tailored for 13.5 nm wavelength to decide the essential height of the zone plate. Then, Monte Carlo simulation method was used to figure out the achievable zone plate parameters by high-resolution electron beam lithography (EBL). Finally, this work systematically explored the viability of nanofabricating top-tier 20 nm resolution hydrogen silsesquioxane (HSQ) zone plates with the duty cycle ratio nearing 1:1 and the aspect ratio approaching 13:1 on 50 nm thick Si3N4 membranes.

Real-time and operando observation of intermediates on TiO2 photoelectrocatalysis by soft X-ray absorption spectroscopy

The oxygen evolution reaction (OER) on the TiO2 surface at the solid–liquid interface was observed in real-time and operando using fluorescence-yield X-ray absorption spectroscopy (XAS) in the soft X-ray region. The OER was observed during a potential sweep with UV light on or off, focusing on the oxygen K-edge XAS. Although conventional XAS measurements require long measurement times, the spectra during the reaction were obtained every 3 s in the present experiment via wavelength-dispersive XAS technique. An increase in the X-ray absorption intensity at approximately 533.8 eV was observed during the potential sweep only with UV light on. The observed spectral change is discussed in relation to the reaction mechanism of the OER. The present technique can be applied to a wide range of analyses of (photo-) electrocatalysis and electrochemical reactions at solid–liquid interfaces to observe their products and intermediates during the reaction.

Impact of B4C buffer layer on interface diffusion in Cr/Sc multilayers: combined study by x-ray reflectivity, scattering and fluorescence

In thin film multilayer based optical componentsof x-ray imaging system, diffusion of one material into the other degrades the reflectivity of the mirrors severely. Along with this thermodynamically driven diffusion, there are also growth generated interface roughness of different special frequencies and microstructures which can increase the diffused scattering from the multilayer and reduce the resolution of an image. Generally grazing incidence x-ray reflectivity in specular geometry (specular GIXR) and diffused x-ray scattering measurement in rocking scan geometry yield information regarding microstructure and overall diffusion at the interfaces of a multilayer. In this paper it is shown that grazing incidence x-ray fluorescence (GIXRF) measurement in standing wave condition alongwith the above measurements can give precise information regarding element-specific diffusion at the interfaces of a multilayer structure. Periodic multilayers made of 75 Cr/Sc bilayers with bilayer thickness ∼4 nm with and without B4C barrier layer of 0.2 nm thickness at the interfaces have been prepared using ion beam sputtering system and characterized by GIXR, diffused x-ray scattering and GIXRF measurements using synchrotron x-ray radiation just above the Cr K-edge. From the above measurements, drastic reduction in interface diffusion of Cr and improvement of interface morphology after addition of B4C barrier layer at the interfaces of Cr/Sc multilayers have been observed which is also corroborated by cross-sectional transmission electron microscopy of the multilayers. Finally, in the water window soft x-ray region of 2.3–4.4 nm performance of these multilayers have been tested and the Cr/B4C/Sc multilayer with improved interface quality has been found to yield ∼30.8% reflectivity at 3.11 nm wavelength which is comparable with the best reported reflectivities in the literature at this wavelength.

Chemical States of Stainless Steel in nm to Several Tens of nm Region from Surface Observed by the Depth-Resolved X-ray Absorption Spectroscopy.

Chemical states of Fe, Cr, Ni, Mn, and O in stainless steel, SS304, are nondestructively observed over a wide range of depth from nanometer (nm) to several tens of nm by means of the depth-resolved X-ray absorption spectroscopy in the soft X-ray region. It is revealed that Cr is more oxidized in the surface region than the inner region, while Fe is also oxidized at the surface though the oxidation is less prominent compared to Cr. The chemical states of Cr are described by a mixture of Cr-O and Cr-OH coordinations, with more OH components at the surface. Mn and Ni spectra show MnO and metallic Ni features, respectively, throughout the depth of at least up to several tens of nm.

Recent advances in high-order harmonic generation from laser-ablated plumes at the advanced laser light source laboratory

High-order harmonic generation is a nonlinear optical frequency conversion process that occurs during intense ultrafast laser-matter interaction. At the Advanced Laser Light Source laboratory, we use ultrafast laser pulses having diverse wavelengths, spanning visible, near- and mid-infrared ranges, to generate high-order harmonics from laser-ablated plumes in the extreme ultraviolet or soft X-ray region of the electromagnetic spectrum. The Advanced Laser Light Source Laboratory is situated within the Énergie Matériaux Télécommunications Center of the Institut national de la recherche scientifique in Montréal, Quebec, Canada. We focus on generating bright and broadband harmonics by exploiting various types of ultrafast resonances in different species within the laser-ablated plume, and use them for applications in ultrafast spectroscopy, imaging, and AMO science. We are also actively exploring previously unknown physics governing the harmonic generation from different resonances. In this review article, we provide an overview of the recent advancements made in these directions.

Soft X-ray observing the cosmic sources by the ESA–CAS satellite SMILE

This paper describes the scientific potential of a Soft X-ray Imager (SXI) onboard the ESA–CAS satellite SMILE for investigating cosmic X-ray sources of various types. We show that this instrument, albeit designed for X-ray imaging of solar wind Charge eXchange of the magnetosheath and the cusps, is also essential for astrophysics because it is able to provide wide-field imaging of the sky in the soft X-ray region. Regarding sufficiently luminous X-ray sources with continuous spectra to be observable by the X-ray monitor MAXI/ISS to assess the object types and their light curves that are expected to be detected by SXI/SMILE, the compact sources accreting matter are promising targets for evaluating the possibilities of SXI. We assumed only the cosmic objects located in the planned fields to be observed by SXI. We used the 2–3keV band flux of MAXI/ISS covering at least part of the expected band of the SXI/SMILE telescope. We used the data obtained by MAXI/ISS to assess the object types and their light curves expected to be detected by SXI/SMILE. We show the long-term activity of the examples of X-ray binary types located in this region. We present the typical features of such objects' activity on long timescales (e.g., outbursts and state transitions) and include physical interpretations of these phenomena. Furthermore, we discuss how SXI can contribute to this branch.

K-shell X-Ray Emission from Lithium-like Nitrogen N v

We present laboratory measurements of n = 3 → n = 1 N v X-ray emission lines situated near 26 Å. The lines are excited by electron-impact collisions and are shown to reach a combined intensity of about a fifth of the combined strong N v 1s2s2p 2 P 1/2,3/2 → 1s 22s 2 S 1/2 resonance lines, commonly labeled q and r, at 29.4 Å. In addition, we present new experimental data for the wavelength of the blended q and r lines at 29.4 Å, as well as for that of the blended inner-shell-excited N v lines u and v at 30.0 Å. All of these collisional N v lines need to be included in astrophysical emission models in order to properly account for flux from N v in the soft X-ray region. The measured wavelengths provide benchmarks for testing atomic structure calculations and excellent agreement is found with our calculations using the many-body perturbation theory method. We provide a complete listing of the N v energy levels with valence electrons in the n = 2, 3, and 4 shells calculated with this approach. The experimental and theoretical data, thus, provide accurate rest-frame wavelengths needed for velocity determinations based on high-resolution absorption features in spectra of warm absorbers in active galactic nuclei and other astrophysical objects.

A versatile beamline for soft x-ray reflectivity, absorption, and fluorescence measurements at Indus-2 synchrotron source.

A versatile beamline for performing reflectivity, fluorescence, and absorption experiments in the soft x-ray region of 100-1500eV is commissioned on a bending magnet port of the Indus-2 synchrotron source. A high vacuum 2-axis reflectometer with x, y, and z sample scanning stages is installed. This reflectometer is used to measure the reflectivity of large samples up to 300mm in length and 5kg in weight. This feature is useful for characterizing x-ray optical elements, such as mirrors, gratings, and multilayers. A flange mounted silicon drift detector is installed in the downstream of the reflectometer for soft x-ray fluorescence measurements. The soft x-ray absorption measurements are carried out in the total electron yield and partial fluorescence yield modes. Integration of three different experimental techniques in the experimental station makes the beamline versatile for materials science applications as it provides structural, chemical, and electronic state information by performing the required experiments in an identical environment. The beamline uses a varied line spacing plane grating monochromator and gives a high flux (∼109 to 1011 photon/s) with a moderate resolution (λ/Δλ ~1000-5000). A three-mirror-based higher harmonic setup is incorporated to get rid of harmonics and to get a high spectral purity monochromatic beam with less than 0.1% harmonic content. In the present article, the beamline optical scheme, mechanical configuration, and details of the experimental setups are presented, along with a few representative results of each experimental mode.

Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV—soft X-ray applications

We demonstrate a novel flat-field, dual-optic imaging EUV—soft X-ray spectrometer and monochromator that attains an unprecedented throughput efficiency exceeding 60% by design, along with a superb spectral resolution of λ/Δλ > 200 accomplished without employing variable line spacing gratings. Exploiting the benefits of the conical diffraction geometry, the optical system is globally optimized in multidimensional parameter space to guarantee optimal imaging performance over a broad spectral range while maintaining circular and elliptical polarization states at the first, second, and third diffraction orders. Moreover, our analysis indicates minimal temporal dispersion, with pulse broadening confined within 80 fs tail-to-tail and an FWHM value of 29 fs, which enables ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. Furthermore, the spectrometer can be effortlessly transformed into a monochromator spanning the EUV—soft X-ray spectral region using a single grating with an aberration-free spatial profile. Such capability allows coherent diffractive imaging applications to be conducted with highly monochromatic light in a broad spectral range and extended to the soft X-ray region with minimal photon loss, thus facilitating state-of-the-art imaging of intricate nano- and bio-systems, with a significantly enhanced spatiotemporal resolution, down to the nanometer–femtosecond level.

Atomic structure calculations, and electron impact ionization cross-sections of W LXI

The present study provides a wide range of calculations of atomic structure data for the tungsten ion W LXI, that belongs to the silicon isoelectronic sequence. The fully relativistic Multi-configuration Dira-Hartree–Fock (MCDHF) method including configuration interaction (CI) is applied to the current ion. Calculated results of energy levels, oscillator strengths, and radiative rates of W LXI are presented. A set of two configuration state lists including the configurations of 3s23p2, 3s3p3, 3s23p3d, and 3p4 at different angular momenta (J) and parity (P) are considered in the configuration interaction calculations of level energies and oscillator strengths. To increase the accuracy of the results, the electronic correlations from higher orbitals, i.e., 7l orbitals are included in the calculations. Quantum electrodynamics effects such as vacuum polarization and self-energy are recalled in the CI calculations. Many more transitions in the soft x-ray region (between 20 and 200 angstroms) have been identified. The flexible atomic code package has been used to compute the electron impact ionization cross-sections of 3s23p2 3P0,1, 1D2 levels in silicon-like tungsten. The cross-sections of the transitions from the ground level in W LXI to the first levels in the 61st ionization of tungsten have been computed using FAC code. The total ionization cross-sections of the 3s23p2 at electron energy range from 5–35 keV are evaluated as well.

A versatile wide energy range spectrometer for soft X-ray FELs

The design and main characteristics of the Wide Energy range Spectrometer at TIMEX (WEST) operating at the FERMI Free Electron Laser are described. WEST covers the entire photon energy range of the FERMI source including its fundamental emission (extreme ultraviolet region, EUV) and its highest harmonics (soft X-ray region) spanning about a thousand of electrovolts from 20 to 1000 eV. The instrument is specifically designed to measure FERMI FEL spectra with good resolving power (500<RP<7000) and provide remarkable signal to noise ratio adequate for single-shot experiments. Notably, WEST is aligned in transmission geometry downstream of the TIMEX experimental beamline thus offering a valid diagnostic tool for the characterization of both linear and nonlinear spectral effects driven by the interaction between FEL radiation and samples in gaseous, liquid and solid phase. Detailed theoretical analysis of the instrument performance complemented by experimental measurements is presented. Further possible upgrades of WEST are finally considered and discussed.

Direct reconstruction of isolated XUV or soft x-ray attosecond pulses from high-harmonic generation streaking spectra

Characterization of an isolated attosecond pulse (IAP) in the extreme ultraviolet (XUV) or soft x-ray (SXR) region is essential for its applications. Here we propose to retrieve an IAP in the time domain directly through the modulation of high-harmonic generation (HHG) spectra in the presence of a time-delayed intense few-cycle infrared or mid-infrared laser. The retrieval algorithm is derived based on the strong-field approximation and an extended quantitative rescattering model. We show that both isolated XUV pulses with a narrow spectral bandwidth and isolated SXR pulses with a broad bandwidth can be well characterized through the HHG streaking spectra. Such an all-optical method for characterizing the IAP differs from the commonly used approach based on the streaked photoelectron spectra that would require electron spectrometers. We check the robustness of the retrieval method by changing the dressing laser or by adjusting the steps of time delay. We also show that the XUV pulse can be accurately retrieved by treating the HHG streaking spectra calculated from solving the time-dependent Schrödinger equation for single atoms as the ‘experimental’ data.

Mass absorption coefficient in the 200-800eV region of sp3 carbon atoms measured using self-standing polyethylene thin films.

To experimentally determine the mass absorption coefficient (μ) of a sp3-carbon (C) atom in the soft X-ray region, soft X-ray absorption spectra (XAS) in the 200-800eV and C K regions of 200-nm-thick self-standing polyethylene (PE) films were measured in the transmission and total electron yield (TEY) modes. PE films were prepared by a spin-coating method. Their experimentally measured thickness and density are 200nm and 0.920g/cm3, respectively. Soft X-ray absorption measurements were performed in beamline BL-6.3.2 at the Advanced Light Source. Although surface oxygen can be slightly observed in the O K and C K regions in TEY-XAS, it cannot be observed in absorbance-XAS. The absorbance-XAS profiles agree well with the calculated profiles, except in the C K threshold. Hence, it can be confirmed that the absorption-XAS measurements are achieved. From the absorbance, μ of sp3-C in PE is 7 × 104 cm2/g near 288eV and 294eV.

Theoretical Investigation of the X-ray Stark Effect in Small Molecules.

We have studied the Stark effect in the soft x-ray region for various small molecules by calculating the field-dependent x-ray absorption spectra. This effect is explained in terms of the response of molecular orbitals (core and valence), the molecular dipole moment, and the molecular geometry to the applied electric field. A number of consistent trends are observed linking the computed shifts in absorption energies and intensities with specific features of the molecular electronic structure. We find that both the virtual molecular orbitals (valence and/or Rydberg) as well as the core orbitals contribute to observed trends in a complementary fashion. This initial study highlights the potential impact of x-ray Stark spectroscopy as a tool to study electronic structure and environmental perturbations at a submolecular scale.

Assessing the Accuracy of Mass Attenuation Coefficients for Soft X-ray EPMA.

The use of soft X-rays in electron probe microanalysis (EPMA) has gained renewed interest over the past decades due to the advent of new detector technologies. Because X-ray absorption is the dominant correction for soft X-rays, a reliable set of mass attenuation coefficients (MACs) is needed for accurate composition determination. Although several MAC tabulations cover the soft X-ray range, the accuracy of such tabulations below 1 keV is not firmly established. In this study, we assess the accuracy of MAC tabulations in the soft X-ray region by comparing tabulated values for Be, B, C, N, O, and F Kα X-rays with experimental data available in the literature. We find that the 1993 semi-empirical MAC compilation of Henke et al. [(1993). Low-energy X-ray interaction coefficients: Photoabsorption, scattering, transmission and reflection at E=50-30000 eV, Z=1-92. Atom Data Nucl Data Tables54, 181-342] and the more recent theoretical MAC calculations of Sabbatucci and Salvat [(2016). Theory and calculation of the atomic photoeffect. Rad Phys Chem121, 122-140] perform slightly better than the rest of the considered tabulations. The Sabbatucci-Salvat dataset also provides the best agreement with the few existing experimental measurements for Al L2,3M X-rays.

Automatic transmission optimization of the plane-grating monochromator using collimated light

It is essential to perform plane-grating monochromator transmission optimization for synchrotron radiation beamlines. Usually, The optimization is time-consuming. In this contribution, an automatic transmission optimization method for the plane-grating monochromator using collimated light based on the Differential Evolution (DE) algorithm is introduced. This method can quickly find the optimal solution. To test this method, a program developed using C++ is verified by optimizing the efficiency of a plane-grating monochromator with a 2000 lines/mm lamellar grating at soft X-ray region. Furthermore, the transmission optimizations of most of the PGM using lamellar gratings in soft x-ray region are performed.