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.