Soft X-ray Region Research Articles

Energy- and spatial-resolved detection using a backside-illuminated CMOS sensor in the soft X-ray region

We recently developed a backside-illuminated CMOS image sensor for soft X-ray detection based on a commercial CMOS sensor. This sensor demonstrated a high quantum efficiency, low readout noise of 2.6 e− rms, and high frame rate of 48 fps. This sensor also showed an energy resolution of 80 eV to 1,000 eV photon. We demonstrated fluorescence detection from highly-oriented graphite and a polymethyl methacrylate film using a simple setup, where the samples faced a CMOS sensor without a monochromator. This energy-resolving performance and high frame rate will facilitate new applications for soft X-ray and EUV experiments.

Broadband nano-focusing of high-order harmonics in soft X-ray region with ellipsoidal mirror

We demonstrate broadband focusing of multiple high-order harmonics of intense femtosecond laser pulses using an ellipsoidal mirror. The ellipsoidal mirror, with a high numerical aperture and a highly accurate surface, was fabricated using a replication process. The multiple high-order harmonics in the wavelength range between 10 and 20 nm were focused to the nearly diffraction-limited size of 350 × 380 nm2. According to Rayleigh's quarter wavelength criteria, the wavefront aberration of the focused beam was estimated to be less than 5 nm, which corresponds to a temporal dispersion of 16 as. The developed focusing system is suited for producing an intense attosecond laser field with negligible wavefront aberration, with which nonlinear light-matter interactions in the attosecond time domain can be explored.

Brilliance improvement of laser-produced extreme ultraviolet and soft x-ray plasmas based on pulsed gas jets

Two methods improving the brilliance of laser-induced plasmas emitting in the extreme UV (EUV) and soft x-ray (SXR) regions were investigated, using three different gases (nitrogen, krypton, and xenon) from a pulsed gas jet. Utilizing a newly designed piezoelectric valve, up to almost ten times higher gas pressures were applied, resulting in increased target densities and thus, higher conversion efficiencies of laser energy into EUV and SXR radiation. Secondly, geometrically reducing the angle between the incoming laser beam and the observed plasma emission minimizes reabsorption of the emitted short wavelength radiation. Combining both methods, the source brilliance is increased by a factor of 5 for nitrogen. Furthermore, a compact EUV focusing system for metrological applications is presented utilizing the optimized plasma source. An energy density of 1 mJ/cm2 at wavelength λ = 13.5 nm in the focal spot of an ellipsoidal mirror is achieved with xenon as the target gas being sufficient for material removal of PMMA samples with an ablation rate of 0.05 nm/pulse.

XUV spectra and soft X-ray laser wavelengths of 4f–4d transitions in $${\hbox{Ni}}^{+8}$$ Ni + 8

For $$3p^{6}3\hbox {d}4l$$ where $$l=s, p, d, f$$ states in $$\hbox {Ni}^{+8}$$ , we evaluated both oscillator strengths and energy levels to investigate the probability of laser wavelengths production in the soft X-ray and XUV regions. The calculations are executed using CIV3 and LANL codes. By comparing the values of our results with other published experimental and theoretical data, a good match is achieved in addition to unpublished values of oscillator strengths and energy levels have been obtained. Using calculated oscillator strengths, the gain coefficients, and reduced populations have been calculated for $$\hbox {Ni}^{+8}$$ at different plasma temperatures, such as $$T = 1/4$$ the ionization potential energy (IP) of the $$\hbox {Ni}^{+8}$$ ; and the same thing for $$T = 1/2\ IP$$ and $$T = 3/4\ IP$$ . Promised values of positive laser gain are shown to obtain laser wavelengths in XUV and soft X-ray regions from current computed atomic data. Extreme ultraviolet radiation XUV is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm.

The Potential of EuPRAXIA@SPARC_LAB for Radiation Based Techniques

A proposal for building a Free Electron Laser, EuPRAXIA@SPARC_LAB, at the Laboratori Nazionali di Frascati, is at present under consideration. This FEL facility will provide a unique combination of a high brightness GeV-range electron beam generated in a X-band RF linac, a 0.5 PW-class laser system and the first FEL source driven by a plasma accelerator. The FEL will produce ultra-bright pulses, with up to 10 12 photons/pulse, femtosecond timescale and wavelength down to 3 nm, which lies in the so called “water window”. The experimental activity will be focused on the realization of a plasma driven short wavelength FEL able to provide high-quality photons for a user beamline. In this paper, we describe the main classes of experiments that will be performed at the facility, including coherent diffraction imaging, soft X-ray absorption spectroscopy, Raman spectroscopy, Resonant Inelastic X-ray Scattering and photofragmentation measurements. These techniques will allow studying a variety of samples, both biological and inorganic, providing information about their structure and dynamical behavior. In this context, the possibility of inducing changes in samples via pump pulses leading to the stimulation of chemical reactions or the generation of coherent excitations would tremendously benefit from pulses in the soft X-ray region. High power synchronized optical lasers and a TeraHertz radiation source will indeed be made available for THz and pump–probe experiments and a split-and-delay station will allow performing XUV-XUV pump–probe experiments.

Narrowband lamellar multilayer grating with low contrast MoSi2/Si materials for the soft x-ray region

To develop highly efficient narrow-bandwidth multilayer optics for the soft x-ray (SXR) spectroscopy, a low optical contrast MoSi2/Si lamellar multilayer grating (LMG) was proposed and developed. The low contrast LMG allows for a large lamel width which can potentially achieve higher resolution than the conventional LMG and simplify the fabrication. As a first demonstration, a MoSi2/Si multilayer with a d-spacing of 5 nm and 180 bilayers was deposited. The lamellar grating structure with a period of 614 nm, lamel-to-period ratio of 0.38, and lamel height of 670 nm was fabricated in the multilayer with reactive ion etching process. The SXR measurements show a high 0th-order diffraction efficiency of 16%–33% at 876 eV–1648 eV, which reaches around 80% of the unetched multilayer reflectivity. A maximal bandwidth reduction of 2.2 times was obtained compared with the multilayer mirror, indicating an energy resolution of E/ΔE = 108 at 1183 eV. The resolution can be further improved by reducing the multilayer d-spacing and increasing the etching depth.

NOVEL ANALYTICAL STUDY FOR REACTION INTERMEDIATES IN THE PRIMARY RADIATION INTERACTION OF DNA USING A SYNCHROTRON RADIATION-INDUCED LUMINESCENCE SPECTROSCOPY

To identify the precise molecular processes to induce DNA lesions, we attempt a novel spectroscopy of X-ray induced luminescence (XIL) using soft X-ray synchrotron radiation, which is a non-destructive analysis of the reaction intermediates in the elementary reaction pathway of damage induction and self-organized restoration. Using a liquid micro-jet technique to introduce aqueous samples in a vacuum chamber, we measure UV-visible luminescence from nucleotide solution as a function of the soft X-ray energy from the nitrogen to oxygen K-edge region. The XIL intensities for the nucleotide solutions are significantly enhanced in the soft X-ray region (410-530 eV) which is ascribed to the K-shell excitation/ionization of nitrogen atoms in the nucleobases. Furthermore, the XIL spectra do not show any signature of X-ray absorption near-edge structure (XANES) of the nucleobases. This is because the luminescence intensities collected from the integral area of the micro-jet only reflect the quantum yield of luminescence of the absorbed X-ray into UV-visible light irrespective of the absorption cross sections, i.e. of XANES. Thus the present result is the first evidence of luminescence as a result of X-ray absorption of aqueous nucleotides.

Study of Relativistic excitation energies and transition data of X–ray and EUV transitions in Be-like ions

In the present work, fine structure energies, transition wavelengths, transition rates, oscillator strengths and line strengths have been presented for lowest 40 levels of Be-like Fe, Os to Hg ions by using GRASP2K. We have analyzed the effects of core-valence (CV) correlations in excitation energies as a function of active set. We accomplish that effects of core-valence correlations are necessary to attain accuracy and convergence in excitation energies. We have compared our presented excitation energies of Be-like Fe with available theoretical and experimental energies and found good agreement. But due to unavailability of data for Be-like Os to Hg ions for comparison, we have calculated excitation energies from flexible atomic code (FAC). We show that magnetic multipole transitions (M2) have effective contribution in lifetime of 2s3p 3P2o and 2p3d 3P2o. We demonstrate that transitions of Be-like ions among lowest 40 levels mainly occurs in Hard X-ray (HXR), Soft X-ray(SXR) and Extreme Ultraviolet (EUV) regions.

REFLEX: a program for the analysis of specular X-ray and neutron reflectivity data

The use of X-ray and neutron reflectivity has been generalized worldwide for scientists who want to determine specific physical properties (such as electron-density profile, scattering-length density, roughness and thickness) of films less than 200 nm thick deposited on a substrate. This paper describes a freeware program namedREFLEX, which is a standalone program dedicated to the simulation and analysis of X-ray and neutron reflectivity from multilayers. This program was first written two decades ago and has been constantly improved since, but never published until now. The latest version ofREFLEXcovers generalized types of calculation of reflectivity curves including both neutron and X-ray reflectivity. In the case of X-rays, the program can deal with both s and p polarization, which is quite important in the soft X-ray region where the two polarizations can yield different results. Neutron reflectivity is calculated within the framework of non-spin-polarized neutrons.REFLEXhas also been designed to include any type of fluid (such as supercritical CO2) on top of the analysed film and includes corrections of the footprint effect for analysis on an absolute scale.

Fast and easy fabrication methodology of Fresnel zone plates for the extreme ultraviolet and soft x-ray regions.

Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15mm are reported: (i)phase-shifting zone plates made by 190nm thick PMMA rings on Si3N4 membranes, (ii)absorbing zone plates made by 75nm thick Au ring structures on Si3N4, and (iii)freestanding Au rings of 50nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9±3 μm, which is the theoretical limit resulting from the synchrotron beam parameters at 60eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance.

Towards GW-Scale Isolated Attosecond Pulse Far beyond Carbon K-Edge Driven by Mid-Infrared Waveform Synthesizer

We discuss the efficient generation of intense “water window” (0.28–0.54 keV) isolated attosecond pulses (IAPs) using a mid-infrared (MIR) waveform synthesizer. Our numerical simulations clearly indicate that not only a longer-wavelength driving laser but also a weak control pulse in the waveform synthesizer helps extend the continuum cutoff region and reduce the temporal chirp of IAPs in high-order harmonic generation (HHG). This insight indicates that a single-cycle laser field is not an optimum waveform for generating the shortest IAP from the veiwpoints of reducing the attochirp and increasing the efficiency of HHG. By combining a waveform synthesizer technology and a 100 mJ MIR femtosecond pulse based on a dual-chirped optical parametric amplification (DC-OPA) method, a gigawatt-scale IAP (55 as with 10 nJ order) in the water window region can be generated even without attochirp compensation. The MIR waveform synthesizer is highly beneficial for generating a shorter IAP duration in the soft X-ray region because there are no suitable transparent dispersive materials that can be used for compressing the attochirp.

Elastic Scattering of Ultrashort Laser Pulses on Noble Gas Atoms

The work deals with a theoretical investigation of elastic scattering of ultrashort electromagnetic pulses (USP) on the noble gas atoms in the far ultra-violet and soft X-ray regions. The investigation is performed within the framework of a dipole approximation and the first-order quantum-mechanics perturbation theory. Major attention is given to the study of the dependence of the total probability of scattering within the pulse time on the USP duration at different carrying frequency values. It is shown that at certain values of the problem parameters this dependence is nonlinear. The calculations are performed for the corrected Gaussian pulses and wavelet pulses without the carrying frequency.

Resonant x-ray diffraction from chiral electric-polarization structures

Heterostructures of ${\mathrm{PbTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattices have shown the formation of ``polar vortices,'' in which a continuous rotation of ferroelectric polarization spontaneously forms. Recently, Shafer et al. [Proc. Natl. Acad. Sci. USA (PNAS) 115, 915 (2018)] reported strong nonmagnetic circular dichroism (CD) in resonant soft x-ray diffraction at the Ti ${L}_{3}$ edge from such superlattices. The authors ascribe the CD to the chiral rotation of a polar vector. However, a polar vector is invisible to the parity-even electric-dipole transition which governs absorption in the soft x-ray region. A realistic, nonmagnetic explanation of the observed effect is found in Templeton-Templeton scattering. Following this route, the origin of the CD in Bragg diffraction is shown by us to be the chiral array of charge quadrupole moments that forms in these heterostructures. While there is no charge quadrupole moment in the spherically symmetric $3{d}^{0}$ valence state of ${\mathrm{Ti}}^{4+}$, the excited state $2{p}^{5}3{d}^{1}({t}_{2g})$ at the Ti ${L}_{3}$ resonance is known to have a quadrupole moment. Our expressions for intensities of satellite Bragg spots in resonance-enhanced diffraction of circularly polarized x rays, including their harmonic content, account for all observations reported by Shafer et al. We predict both intensities of Bragg spots for the second harmonic of a chiral superlattice and circular polarization created from unpolarized x rays, in order that our successful explanation of existing diffraction data can be further scrutinized through renewed experimental investigations. The increased understanding of chiral dipole arrangements could open the door to switchable optical polarization.

Fabrication of a precise ellipsoidal mirror for soft X-ray nanofocusing.

In X-ray focusing, grazing incidence mirrors offer advantages of no chromatic aberration and high focusing efficiency. Although nanofocusing mirrors have been developed for the hard X-ray region, there is no mirror with nanofocusing performance in the soft X-ray region. Designing a system with the ability to focus to a beam size smaller than 100 nm at an X-ray energy of less than 1 keV requires a numerical aperture larger than 0.01. This leads to difficulties in the fabrication of a soft X-ray focusing mirror with high accuracy. Ellipsoidal mirrors enable soft X-ray focusing with a high numerical aperture. In this study, we report a production process for ellipsoidal mirrors involving mandrel fabrication and replication processes. The fabricated ellipsoidal mirror was assessed under partial illumination conditions at the soft X-ray beamline (BL25SU) of SPring-8. A focal spot size of less than 250 nm was confirmed at 300 eV. The focusing tests indicated that the proposed fabrication process is promising for X-ray mirrors that have the form of a solid of revolution, including Wolter mirrors.

Cryo scanning transmission x-ray microscope optimized for spectrotomography.

A cryo scanning transmission X-ray microscope, the cryo-STXM, has been designed and commissioned at the Canadian Light Source synchrotron. The instrument is designed to operate from 100 to 4000 eV (λ = 12.4 - 0.31 nm). Users can insert a previously frozen sample, through a load lock, and rotate it ±70° in the beam to collect tomographic data sets. The sample can be maintained for extended periods at 92 K primarily to suppress radiation damage and a pressure on the order of 10-9 Torr to suppress sample contamination. The achieved spatial resolution (30 nm) and spectral resolution (0.1 eV) are similar to other current soft X-ray STXMs, as demonstrated by measurements on known samples and test patterns. The data acquisition efficiency is significantly more favorable for both imaging and tomography. 2D images, 3D tomograms, and 4D chemical maps of automotive hydrogen fuel cell thin sections are presented to demonstrate current performance and new capabilities, namely, cryo-spectrotomography in the soft X-ray region.

X-ray binary Beta Lyrae and its donor component structure

AbstractWe show that the structure of magnetized accretion gas flows between the components of the Beta Lyrae system can cause a scattering gas shell that masks completely these components in soft X-ray region. Also we have calculated the inner structure of the donor that is filling a Roche lobe and is preceding a forming of the degenerate dwarf. We show that mass of the degenerate core of the donor is in region 0.3−0.5M⊙.

Enhanced high-harmonic generation up to the soft X-ray region driven by mid-infrared pulses mixed with their third harmonic.

We systematically study the efficiency enhancement of high-harmonic generation (HHG) in an Ar gas cell up to the soft X-ray (SXR) range using a two-color laser field composed of 2.1 μm (ω) and 700 nm (3ω) with parallel linear polarization. Our experiment follows the recent theoretical investigations that determined two-color mid-infrared (IR) pulses, mixed with their third harmonic (ω + 3ω), to be close to optimal driving waveforms for enhancing HHG efficiency in the SXR region [Jin et al., Nature Comm. 5, 4003 (2014)]. We observed sub-optical-cycle-dependent efficiency enhancements of up to 8.2 of photon flux integrated between 20 - 70 eV, and up to 2.2 between 85 - 205 eV. Enhancement of HHG efficiency was most pronounced for the lowest tested backing pressure (≈ 140 mbar), and decreased monotonically as the pressure was increased. The single-color (ω)-driven HHG was optimal at the highest backing pressure tested in the experiment (≈ 375 mbar). Our numerical simulations based on single-atom response and 3D pulse propagation show good qualitative agreement with experimental observations. The lower enhancement at high pressure and higher photon energy indicates that phase matching of two-color-driven HHG is more sensitive to ionization rate and pulse propagation effects than the single-color case. We show that with further improvements to the relative phase jitter and the spatio-temporal overlap of the two beams, the efficiency enhancement could be further improved by at least a factor of ≈ 2.

Optical performance of W/B4C multilayer mirror in the soft x-ray region

W/B4C x-ray multilayers (MLs) with 300 layer pairs and a period in the range of d = 2–1.6 nm are fabricated and investigated for the x-ray optical element in the soft x-ray regime. The structural analyses of the MLs are carried out by using hard x-ray reflectivity (HXR) measurements at 8.047 keV. Well-defined successive higher order Bragg peaks (up to 3rd order) in HXR data collected up to glancing incidence angles of ∼9° reveal a good quality of the periodic structure. The ML mirrors have an average interface width of ∼0.35 nm and have a compressive residual stress of ∼0.183 GPa and 0. 827 GPa for d = 1.62 nm and d = 1.98 nm, respectively. MLs maintain structural stability over a long time, with a slight increase in interface widths of the W layers by 0.1 nm due to self-diffusion. Soft x-ray reflectivity (SXR) performances are evaluated in the energy range of 650 to 1500 eV. At energy ∼ 1489 eV, measured reflectivities (energy resolution, ΔE) are ∼ 10% (19 eV) and 4.5% (13 eV) at glancing incident angles of 12.07° and 15° for MLs having periods of 1.98 nm and 1.62 nm, respectively. The optical performance from 1600 eV to 4500 eV is theoretically analysed by considering the measured structural parameters. The structure-stress-optical performance is correlated on the basis of the mechanism of film growth. The implications of W/B4C MLs are discussed, particularly with respect to the development of ML optics with high spectral selectivity and reflectance for soft x-ray instruments.

Thin-film X-ray filters on microstructured substrates and their thermophysical properties

It is shown that structured substrates having micron- or submicron-sized through holes and coated with an ultrathin organic film can be used for the fabrication of thin-film X-ray filters via direct growth of functional layers on a substrate by sputter deposition, without additional complex processing steps. An optimised process is considered for the fabrication of X-ray filters on support structures in the form of electroplated fine nickel grids and on track-etched polymer membranes with micron- and submicron-diameter through pores. ‘Optimisation’ is here taken to mean matching the sputter deposition conditions with the properties of substrates so as to avoid overheating. The filters in question are intended for both imaging and single-channel detectors operating in the soft X-ray and vacuum UV spectral regions, at wavelengths from 10 to 60 nm. Thermal calculations are presented for the heating of ultrathin layers of organic films and thin-film support substrates during the sputter deposition of aluminium or other functional materials. The paper discusses approaches for cooling thin-film composites during the sputter deposition process and the service of the filters in experiments and gives a brief overview of the works that utilised filters produced by the described technique on microstructured substrates, including orbital solar X-ray research in the framework of the CORONAS programme and laboratory laser plasma experiments.

Development of a spectro-electrochemical cell for soft X-ray photon-in photon-out spectroscopy.

We developed a spectro-electrochemical cell for X-ray absorption and X-ray emission spectroscopy, which are element-specific methods to study local electronic structures in the soft X-ray region. In the usual electrochemical measurement setup, the electrode is placed in solution, and the surface/interface region of the electrode is not normally accessible by soft X-rays that have low penetration depth in liquids. To realize soft X-ray observation of electrochemical reactions, a 15-nm-thick Pt layer was deposited on a 150-nm-thick film window with an adhesive 3-nm-thick Ti layer for use as both the working electrode and the separator window between vacuum and a sample liquid under atmospheric pressure. The designed three-electrode electrochemical cell consists of a Pt film on a SiC window, a platinized Pt wire, and a commercial Ag|AgCl electrode as the working, counter, and reference electrodes, respectively. The functionality of the cell was tested by cyclic voltammetry and X-ray absorption and emission spectroscopy. As a demonstration, the electroplating of Pb on the Pt/SiC membrane window was measured by X-ray absorption and real-time monitoring of fluorescence intensity at the O 1s excitation.