小池 雅人

Multilayer diffraction gratings are designed to improve the detection limit and sensitivity of soft x-ray flat-field spectrographs in the region of 300–1000 eV, placing emphasis on Fe-L (705 eV), Cu-L (930 eV), and Zn-L (1012 eV) emissions. For this purpose, spectral flux was used as the performance index, which is proportional to the amount of optical flux incident into a detector and correlated with detection sensitivity. A super-mirror-type W/B4C multilayer coating [Koike et al., Rev.Sci. Instrum. 94, 045109 (2023)] was employed to improve diffraction efficiency in a wide energy region. The unique feature of the multilayer structure is that the average refractive index and the period length of W/B4C layer pairs are increased from the bottom to top layers. In addition, the incidence angle was reduced to 86.03° from 88.65° and the nominal groove density was increased to 3200 lines/mm from 2400 lines/mm of our previous design, to improve spectral flux while maintaining dispersion and spectral resolution. A holographic varied-line-spacing spherical grating and a soft x-ray flat-field spectrograph were designed, using the aspherical-wavefront-recording method, assuming the nominal grating constant and incident angle described above. The numerical simulation results showed that the spectrograph employing the newly designed grating with the W/B4C multilayer indicated 3.2–8.2 times higher spectral flux compared with those using the previously designed grating while keeping the same spectral resolution.

A soft x-ray varied-line-spacing (VLS) laminar-type spherical grating with a super-mirror-type (SMT) multilayer was designed for a soft x-ray high resolution flat-field spectrograph in a region of 2–4 keV. The effective groove density of the designed VLS grating is 3200 lines/mm, and the local groove density varies from 2700 to 3866 lines/mm. The geometrical imaging property was evaluated by numerical calculations. The resolving power estimated by means of ray tracing was up to ∼103. For the evaluation of diffraction efficiency, the SMT multilayer structure designed for 3200 lines/mm in our previous work, Koike et al., Rev. Sci. Instrum. 94, 045109 (2023), was employed, and the numerical calculation was performed considering the local groove density of VLS grooves and the local incidence angle being affected by the curvature of the spherical surface and the geometrical relation between the source and incidence point on the grating. The results showed that the SMT multilayer-coated grating exhibited about an order of magnitude higher diffraction efficiency compared with an Au-coated grating.

Laminar-type spherical diffraction gratings overcoated with carbon-based materials were designed, fabricated, and evaluated for the purpose of enhancing the analytical sensitivity of the flat-field spectrograph in a vacuum ultraviolet region of 35–110 eV. As the design benchmark for numerical calculations, diffraction efficiency (DE) and spectral flux, which are defined by the product of the DE and numerical aperture and correlate with the analytical sensitivity of the spectrograph, were used. To simplify the feasibility study on the overcoating effects, we assumed a laminar-type grating having a grating constant of 1/1000 mm and coated with a Au layer of 30.0 nm thickness and an incidence angle of 84.0°. The optimized groove depth and duty ratio were 30.0 nm and 0.3, respectively. In addition, the optimum thicknesses of the overcoating layer were 44, 46, 24, and 30 nm for B4C, C, diamond-like-carbon, and SiC, respectively. Based on these results, we have fabricated a varied-line-spacing holographic grating overcoated with B4C with a thickness of 47 nm. For the experimental evaluation, we used the light source of Mg-L and Al-L emissions excited by the electron beam generated from an electron microscope, an objective flat-field spectrograph, and a CCD imaging detector. The experimental results showed that the spectrograph employing a new grating overcoated with the B4C layer indicated almost the same spectral resolution and 2.9–4.2 times higher analytical sensitivity compared with those obtained with a previously designed Au-coated grating having a grating constant of 1/1200 mm and used at an incidence of 86.0°.

Soft x-ray diffraction gratings coated with a supermirror-type multilayer were designed to enhance diffraction efficiency in the energy range of 2–4 keV by means of numerical calculations. The optimized groove depth and incidence angle are 2.05 nm and 88.65°, respectively, for the grating having a groove density of 3200 grooves/mm. Regarding the multilayer structure, the optimum number of B₄C/W layers pair was 11 and the thickness of B₄C was increased from bottom to top, while that of W was kept constant. The replacement of the top layer of W by either Co, Cr, or Ni was an effective means of obtaining uniform diffraction efficiency. In the region of 2–4 keV, the calculated diffraction efficiency of the designed gratings was up to ∼5.3%, on average, and almost eight times larger than that of ∼0.7% of an Au coated grating.

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Efficiently detecting the B-K emission band around 6.76 nm from a trace concentration of boron in steel compounds has motivated a theoretical exploration of means of increasing the diffraction efficiency of a laminar grating with carbon overcoating. To experimentally evaluate this enhancement, a Ni grating was coated with a high-density carbon film, i.e., diamond-like carbon (DLC). The first order diffraction efficiencies of the Ni gratings coated with and without DLC were measured to be 25.8 % and 16.9 %, respectively, at a wavelength of 6.76 nm and an angle of incidence of 87.07 degrees. The ratio of diffraction efficiency obtained experimentally vs. that calculated by numerical simulation is 0.87 for the DLC-coated Ni grating. The diffraction efficiency of a Ni grating coated with a low-density carbon film, amorphous carbon (a-C), was also slightly improved to be 19.6 %. Furthermore, a distinct minimum of the zeroth order lights of the two carbon-coated Ni gratings were observed at around 6.76 nm, which is coincident with the maximum of the first order light.

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Boron is the critical trace element in the production of high quality steel, creating a great demand for an efficient detection method of the B-K emission band at around 6.76 nm. To meet this demand we made a simulation study and obtained a practical method to improve the diffraction efficiency of metal-coated laminar-type gratings for a grazing incidence flat-field spectrograph by overcoating a sufficiently transparent high-density material. In the simulation the diffraction efficiency in a spectral region of 3.5 8.5 nm was computed for several combinations of overcoating materials and coating metals, with various thicknesses of the overcoating layer. The result obtained are: (1) the best overcoating material is high-density diamond-like carbon (DLC) having a density of 3.1 g/cm(3), (2) its optimum thickness is 24 nm at an angle of incidence of 87.0, and (3) with this thickness the first-order diffraction efficiency is expected to reach 29.7 %, which well exceeds 15.6 % for Ni-coated (or 14.1 % for Au-coated) grating.

Laser-driven plasma soft X-ray laser, XRL, at a wavelength of 13.9 nm is generated from nickel-like silver plasmas. The polarization state at an end station is considered to be vertically linearly polarized due to the reflections at some Mo/Si multilayer mirrors installed in the XRL beamline, but the detail has not been verified experimentally. To evaluate and control the polarization state, a compact polarization analysis apparatus to adapt for the XRL end station is developed. Two Mo/Si multilayer mirrors are fabricated and the polarization properties are evaluated by using synchrotron radiation, SR. As a preliminary test of the apparatus, the reflectivity of the multilayer is measured by the XRL and it shows good agreement with that by SR. (C) 2014 Elsevier B.V. All rights reserved.

Electron-beam-induced soft-X-ray emission spectroscopy (SXES) that uses a grating spectrometer has been introduced to a conventional scanning electron microscope (SEM) for characterizing desired specimen areas of bulk materials. The spectrometer was designed as a grazing incidence flat-field optics by using aberration corrected (varied line spacing) gratings and a multichannel plate detector combined with a charge-coupled device camera, which has already been applied to a transmission electron microscope. The best resolution was confirmed as 0.13 eV at Mg L-emission (50 eV), which is comparable with that of recent dedicated electron energy-loss spectroscopy instruments. This SXES-SEM instrument presents density of states of simple metals of bulk Mg and Li. Apparent band-structure effects have been observed in Si L-emission of Si wafer, P L-emission of GaP wafer, and Al L-emissions of intermetallic compounds of AlCo, AlPd, Al2Pt, and Al2Au.

A new multilayer-coated varied line-spaced grating, JS4000, was fabricated and tested for extending the upper limit of a grating X-ray spectrometer for electron microscopy. This grating was designed for 2-3.8 keV at a grazing incidence angle of 1.35°. It was revealed that this new multilayer structure enables us to take soft-X-ray emission spectra continuously from 1.5 to 4.3 keV at the same optical setting. The full-width at half maximum of Te-L α1,2 (3.8 keV) emission peak was 27 eV. This spectrometer was applied to indium tin oxide particles and clearly resolved Sn-L α (3444 eV) and In-Lβ1 (3487 eV) peaks, which could not be resolved by a widely used energy-dispersive X-ray spectrometer. © The Author 2013. Published by Oxford University Press [on behalf of Japanese Society of Microscopy]. All rights reserved.

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A flat-field spectrograph equipped with a wide-band multilayer grating and prefocusing mirror covering 2-4 keV without any mechanical movement has been developed. To realize this, a new multilayer structure consisting of W and B4C layers has been invented, which enhances the diffraction efficiency of the grating over the whole energy range at a fixed angle of incidence as well as the reflectivity of the prefocusing mirror. The multilayer has been deposited on a laminar-type varied-line-spacing holographic replica grating and a spherical mirror substrate. The diffraction efficiency of the multilayer grating varies between 1.2% and 3.3% at 88.65 degrees in the 2.1-4.0 keV range. Also the reflectivity of the prefocusing mirror varies between 2.7% and 12% at 88.00 degrees in the same range. The overall throughput of the spectrograph with the multilayer optics is 10(4) times higher than that of gold-coated optics.

We have developed an objective soft x-ray flat-field spectrograph installed in electron microscopes (EMs). The spectrograph has two attractive features. One is that it is designed to cover a wide energy range of 50-4000 eV by using four varied-line-spacing holographic gratings (VLSHGs) optimized for 50-200 eV, 155-350 eV, 300-2200 eV, and 2000-4000 eV. The gratings dedicated for the respective energy ranges can be accommodated in the single spectrograph. This advantage comes from that the positions of the source points and image planes are assumed as the common parameters in the design of all gratings. Therefore, it allows to easily change the energy range by only choosing an appropriate grating and its position. The other is the application of a newly invented W/B4C multilayer coating. It has been adopted to the grating for the 2000-4000 eV range to overcome the considerable decrease of the diffraction efficiency in the energy range above similar to 2 keV. The novel coating makes it possible to enhance uniformly the diffraction efficiency at a constant incidence angle in the whole energy range.

Laminar and blazed type holographic varied-line-spacing spherical gratings for use in a versatile soft x-ray flat-field spectrograph attached to an electron microscope are designed, fabricated, and evaluated. The absolute diffraction efficiencies of laminar (or blazed) master and replica gratings at 86.00 degrees incidence evaluated by synchrotron radiation show over 5% (or 8%) in the 50-200 eV range with the maxima of 22% (or 26%-27%). Also the resolving power evaluated by a laser produced plasma source is in excess of 700 at the energy near the K emission spectrum of lithium (similar to 55 eV) for all gratings. Moreover, the K emission spectrum of metallic Li with high spectral resolution is successfully observed with the spectrograph attached to a transmission electron microscope. (c) 2012 Optical Society of America

A new grating (JS50XL) for ultrasoft-X-ray spectroscopy in an energy range 50-200 eV was designed, manufactured and tested. A spectrometer composed of the grating and a multi-channel plate detector was constructed. At the low-energy end of this spectrometer, a sharp Fermi edge of Mg-L emission was observed at 49.5 eV with an energy resolution of 0.15 eV. Li-K emission spectra were obtained from metal-Li, surface-oxidized metal-Li and 5% Li-Al. Relative energy shifts observed in Al-L emission spectra of Al, AlN and MgAl2O4 were explained by shifts in core binding energies (chemical shift) and bandgap energies of those materials. Si-L emissions from Si, SiC and SiO2 (quartz) and P-L emissions from GaP and InP were presented. Furthermore, the grating was tilted to extend the lower limit of detection energy to 32 eV, and the whole intensity distribution of Mg-L emission was successfully obtained. These ultrasoft-X-ray emission spectra show a successful extension to lower-energy range using the new soft-X-ray emission spectroscopy instrument in electron microscopy.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012. © 2012, Microscopy Society of America. All rights reserved.

Evaluation Beamline for Soft X-Ray Optical Elements (BL-11) at the SR Center of Ritsumeikan University has been operated to measure the wavelength and angular characteristics of soft x-ray optical components in a wavelength range of 0.65-25 nm using a reflecto-diffractometer (RD). The beam intensity monitor that has been equipped in BL-11 has observed the signal of the zero-th order light. For the purpose of more accurate evaluation of the performance of optical components, a new beam intensity monitor to measure the intensity of the first order light from the monochromator in BL-11 has been developed and installed in just front of RD. The strong positive correlation between the signal of the beam monitor and a detector equipped in the RD is shown. It is successful that the beam intensity of the first order light can be monitored in real time.

A multilayer mirror with a novel layer structure to uniformly enhance the reflectivity in a few keV energy range at a fixed angle of incidence is invented and applied to a multilayer grating for use in a flat-field spectrograph attached to a conventional electron microscope. The diffraction efficiency of the fabricated multilayer grating having the new layer structure is evaluated at the angle of incidence of 88.65 degrees in the energy region of 2.1-4.0 keV. It is shown that the multilayer grating is effective to uniformly enhance the diffraction efficiency and able to be practically used in this energy region.

Constant deviation condition performing scanning energy by the rotation of grating around itself is the one of the most practical mounting for grating monochromators. Conventional multilayer coating can obtain high diffraction efficiency merely in a narrow band width. It is found that a new modified multilayer structure on the laminar-type grating intended to enhance the diffraction efficiency for constant deviation condition shows the diffraction efficiency over 5 % in a wide band range of 1.5-2.5 keV in calculation.

A dedicated apparatus for polarization measurements in a soft x-ray region, which was previously developed and installed in a soft x-ray beamline (BL-11) at SR Center, Ritsumeikan University, is thoroughly improved. It allows us to adjust and set the angle of incidence and the height position of a polarization optical component more accurately even in vacuum. Using two Mo/Si multilayer mirrors fabricated as polarizers, the linear polarization degree of a 13.9-nm radiation from the BL-11 beamline is evaluated and determined to be 91%, as well as the polarization performance of the polarizers used.

We have been developing a wavelength-dispersive soft x-ray spectrograph covering an energy region of 50-4000 eV to attach to a conventional electron microscope. Observation of soft x-ray emission in the 2-4 keV range needs a multilayer coated grating. In order to evaluate the performance of the optical component in the energy region, a goniometric apparatus has been newly developed and the preliminary performance has been tested using synchrotron radiation.

© Canopus Academic Publishing Limited 2011. This paper gives an overview of recent progress in the study of laser-driven plasma x-ray lasers in Japan Atomic Energy Agency (JAEA). Fully spatial coherent plasma soft x-ray laser (SXRL) at 13.9 nm with 0.1 Hz repetition rate is now routinely used in the wide variety of the applications: The highlights of these applications are the study of fluctuation in the atomic structure of ferroelectric substances under the phase transition using double SXRL probe technique and the first observation of surface dynamics of laser ablation with 10 ps-time and 1 nm-depth resolution using a single-shot SXRL interferometer.

A versatile soft x-ray flat-field grating spectrograph to be installed to a conventional transmission electron microscope has been developed. A holographic spherical grating of a 1200-lines/mm effective groove density which places emphasis on the low energy region of 50-200 eV is designed by an aspheric wavefront recording system. Laminar and blazed types master (LM and BM) gratings and their respective replica (LR and BR) gratings are fabricated by holographic exposure and ion-beam etching methods. Absolute diffraction efficiencies in the 50-300 eV range at the angle of incidence of 86.0 degrees were measured using a synchrotron radiation. The first order diffraction efficiencies are 6.1-7.5% (or 12%) for LM (or BM) and 7.4-9.6% (or 13%) for LR (or BR) gratings at near 55 eV, and over 5% (or 8%) in the 50-200 eV range for LM and LR (or BM and BR) gratings. The replica gratings show the comparable first-order diffraction efficiencies with their respective laminar and blazed types of master gratings.

Two types of wavelength-dispersive soft X-ray spectrometers, a high-dispersion type and a conventional one, for transmission electron microscopes were constructed. Those spectrometers were used to study the electronic states of valence electrons (bonding electrons). Both spectrometers extended the acceptable energy regions to higher than 2000 eV. The best energy resolution of 0.08 eV was obtained for an Al L-emission spectrum by using the high-dispersion type spectrometer. By using the spectrometer, C K-emission of carbon allotropes, Cu L-emission of Cu(1-x)Zn(x) alloys and Pt M-emission spectra were presented. The FWHM value of 12 eV was obtained for the Pt M alpha-emission peak. The performance of the conventional one was also presented for ZnS and a section specimen of a multilayer device. W-M and Si-K emissions were clearly resolved. Soft X-ray emission spectroscopy based on transmission electron microscopy (TEM) has an advantage for obtaining spectra from a single crystalline specimen with a defined crystal setting. As an example of anisotropic soft X-ray emission, C K-emission spectra of single crystalline graphite with different crystal settings were presented. From the spectra, density of states of pi- and Sigma-bondings were separately derived. These results demonstrated a method to analyse the electronic states of valence electrons of materials in the nanometre scale based on TEM.

A novel apparatus for polarization measurement in the soft X-ray region has been designed, constructed, and installed in the evaluation beamline for soft X-ray optical elements (BL-11) at the SR Center of Ritsumeikan University, Shiga, Japan. It allows us to perform conventional reflection and transmission measurements including rocking curve measurement as well as polarimetric and ellipsometric measurements based on the rotating-analyzer method by using six independently movable motorized stages. As a preliminary test of the apparatus, the reflection profile of a Mo/SiO(2) multilayer mirror prepared by an ion beam sputtering technique, which is designed as a reflection polarizer for use of 13.9 nm, has been measured by the apparatus. The result is compared with that by an existing reflectometer, and the azimuth angle dependence of the reflection intensity has been demonstrated. Consequently, it is shown that the apparatus has the capability to perform the rotating-analyzer ellipsometry. (C) 2009 Elsevier B.V. All rights reserved.

Linear polarization measurements were carried out using a newly developed soft x-ray polarimeter and ellipsometer (SXPE) for complete polarization analysis at a soft x-ray beamline (BL-11) of the SR Center, Ritsumeikan University, Japan. A Mo/Si multilayer mirror (period: 10.36 nm, ratio of Mo layer to period: 0.64, and 23 bilayers with topmost Si layer) was deposited on the surface of a commercial Si(111) substrate by an ion beam sputtering method. It was cut into two pieces which were then used as reflection-type polarizers. When the incident wavelength was scanned from 12.4 nm to 14.8 nm by a Monk-Gillieson type varied-line-spacing grating-monochromator of the BL-11, the angles of incidence of both the Mo/Si multilayer polarizers in the SXPE were also varied from 37.5 degrees to 52.3 degrees. The polarizances depended strongly on the wavelength, and the best performance of over 99% was obtained in the vicinity of 14 nm. Using these polarizers, we assessed that the degree of linear polarization of the BL-11 was almost constant at 85-88% in the measured wavelength range.

W/C and Co/SiO(2) multilayer laminar-type holographic plane gratings (groove density 1/sigma=1200 lines/mm) in the 1-8 keV region are developed. For the Co/SiO(2) grating the diffraction efficiencies of 0.41 and 0.47 at 4 and 6 keV, respectively, and for the W/C grating 0.38 at 8 keV are observed. Taking advantage of the outstanding high diffraction efficiencies into practical soft X-ray spectrographs a Mo/SiO(2) multilayer varied-line-spacing (VLS) laminar-type spherical grating (1/sigma=2400 lines/mm) is also developed for use with a flat field spectrograph in the region of 1.7 keV. For the Mo/SiO(2) multilayer grating the diffraction efficiencies of 0.05-0.20 at 0.9-1.8 keV are observed. The FWHMs of the measured line profiles of Hf-M alpha(1)(1644.6 eV), Si-K alpha(1)(1740.0 eV), and W-M alpha(1) (1775.4 eV) are 13.7 eV, 8.0 eV, and 8.7 eV, respectively. (C) 2009 Elsevier B.V. All rights reserved.

A new apparatus for polarimetric and ellipsometric measurements based on the rotating-analyzer method in the soft x-ray region has been designed, constructed, and installed in the soft x-ray beamline (BL-11) at the SR Center of Ritsumeikan University, Shiga, Japan. It can realize the optical configurations for the complete polarization analysis by using six independently movable drive shafts. A demonstration of the capabilities of the apparatus has been performed using Mo/Si multilayer polarizers deposited by an ion beam sputtering method. It is for the first time shown that the degree of linear polarization of monochromatized light from the BL-11 is approximately 87% at 92 eV since the beamline has been constructed. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3204458]

We demonstrate the generation of greater than l010 temporal contrast with over 1020 W/cm2 peak intensity pulses at a 10 Hz repetition rate from a Ti:sapphire chirped-pulse amplification (CPA) laser system utilizing double CPA, optical parametric chirped-pulse amplification (OPCPA), and cryogenic-cooling techniques. We also successfully produce uncompressed output pulse energy exceeding 30 J, indicating potential for peak power at the 500 TW level. © 2009 American Institute of Physics.

This paper gives an overview of recent progress of laser-driven plasma x-ray lasers in Japan Atomic Energy Agency (JAEA). Fully spatial coherent plasma x-ray laser (XRL) at 13.9 nm with 0.1 Hz repetition rate has been developed using new driver laser system TOPAZ, and the succeeding optimization of the pumping condition has realized more efficient generation of the coherent x-ray pulse. The 0.1 Hz XRL is now routinely used in the wide variety of the application experiments: The highlights of these applications are the study of fluctuation in the atomic structure of ferroelectric substances under the phase transition using the double XRL probe beam technique and the construction of new x-ray laser interferometer to observe nano-scale dynamics of materials. © 2009 SPIE.

The multilayer gratings were fabricated by depositing the Co/Si and Co/SiO(2) multilayers onto the surface of laminar-type holographic gratings having shallow grooves. The structures of these multilayer gratings observed by a transmission electron microscope were the well defined structures without serious defects. The structural property evaluated by comparing the measured and calculated diffraction conditions, i.e., incidence and diffraction angles, resulted that the diffraction conditions of multilayer gratings were affected by the refractive indices of multilayer coatings even at the photon energy of 8.05 keV. The measured low diffraction efficiencies of the Co/Si multilayer grating in the photon energy range of 1-2 keV can be attributed to the large interdiffusion in the Co/Si multilayer coating. (c) 2008 American Institute of Physics. [DOI:10.1063/1.2990062]

A multilayer laminar-type holographic grating having an average groove density of 2400 lines/mm is designed and fabricated for use with a soft-x-ray flat-field spectrograph covering the 0.70-0.75 nm region. A varied-line-spaced groove pattern is generated by the use of an aspheric wavefront recording system, and laminar-type grooves are formed by a reactive ion-etching method. Mo/SiO2 multilayers optimized for the emission lines of Hf-M, Si-K, and W-M are deposited on one of the three designated areas on the grating surface in tandem. The measured first-order diffraction efficiencies at the respective centers of the areas are 18%-20%. The flat-field spectrograph equipped with the grating indicates a spectral linewidth of 8-14 eV for the emission spectra generated from electron-impact x-ray sources. (c) 2007 Optical Society of America.

The thermal stability of Co/SiO2 multilayers was evaluated. Multilayer samples were deposited on Si substrates by means of the ion beam sputtering method and annealed at temperatures from 100 degrees C to 600 degrees C in a vacuum furnace. For the structural and optical evaluations, small-angle x-ray diffraction measurements, soft x-ray reflectivity measurement in the 1 keV energy region, and transmission electron microscopy observations were carried out. As the results, the Co/SiO2 multilayers annealed up to 400 degrees C maintained the initial multilayer structure, and kept almost the same soft x-ray reflectivity as the as-deposited sample. A deterioration of the multilayer structure caused by the growth of cobalt grains was found on the samples annealed over 500 degrees C, and the soft x-ray reflectivity dropped in accordance with the deterioration of the multilayer structure. (C) 2007 American Institute of Physics.

A W/C multilayer laminar-type holographic grating was fabricated and its diffraction efficiency was evaluated in the 1-8 keV region. Taking advantage of its high diffraction efficiency a monochromator equipped with a multilayer varied-line-spacing plane grating was designed. The throughput of the monochromator based on the experimental diffraction efficiency of the multilayer grating and resolving power were calculated in comparism with those of a monochromator equipped with a Au coated grating.