T. Imazono, M. Koike, T. Kawachi, N. Hasegawa, M. Koeda, T. Nagano, H. Sasai, Y. Oue, Z. Yonezawa, S. Kuramoto, M. Terauchi, H. Takahashi, N. Handa, T. Murano
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 ~2 keV. The novel coating makes it possible to enhance uniformly the diffraction
efficiency at a constant incidence angle in the whole energy range.
We have developed a soft x-ray laser (SXRL) interferometer capable of the single-shot imaging of nano-scaled
structure dynamics. The interferometer consisted of the reflection optics including double Lloyd's mirrors and focusing
optics, and the interference fringes are produced on the detector surface. The depth and lateral resolutions of the
interferometer were about 1 nm and 1.8 μm, respectively. By using this interferometer, the initial stage (~50 ps) of the
ablation process of the Pt surface pumped by a 70 fs Ti:Sapphire laser pulse was observed. The expansion speed of the
surface estimated from the result (34 nm/50 ps) indicated that the nano-bubble structures were formed in the initial stage
of the ablation. In order to observe the detailed dynamics, the temporal synchronization between the pump and probe
pulses was improved to be 3 ps by adopting a portion of the SXRL and pump beams as the time fiducials, to which the
pump and probe timing was adjusted by using the x-ray streak camera.
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.
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.
An aspheric wave-front recording system was designed to produce a holographic grating for use in a standard soft X- ray flat field spectrograph interchangeable with a mechanically ruled varied-line-spacing (VLS) grating. The grating grooves recorded with the designed aspheric wave- front recording system were processed to form a laminar groove profile by means of reactive ion etching. Measurements done with synchrotron radiation and a laboratory X-ray source are reported for this laminar-type grating and a commercial grating replicated from a mechanically ruled VLS grating that was specifically designed and fabricated for the standard soft X-ray flat- field spectrography. The laminar-type holographic grating is found to have an absolute first-order efficiency of approximately 10% for wavelengths of approximately 4.5 - 12 nm. It is also shown that the holographic grating is effective in suppressing the higher orders and stray-light level for soft X-ray of 4.36 nm (C-K) and has a comparable spectral resolution to the replica VLS grating.
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