Fig. 4-10 Surface profile of the new grating obtained by atomic force microscope The grating is referred to as a laminar-type holographic grating after the rectangular surface profile and the fabrication by the use of the optical interference of laser beams. A very clear rectangular surface profile is observed.

Fig. 4-11 Evaluation of absolute diffraction efficiency of the new laminar-type holographic grating by synchrotron radiation (curves H). For a comparison, curves for a commercial mechanically ruled grating are also shown (curves M). When a grating is illuminated with soft x-rays of a given energy, it generates not only the usable diffraction light (the first order light, m=1) but also higher order light (m=2,3,...). In the whole energy region measured, the intensity of the higher order light is lower for the new grating (H, m=2) than for the mechanically ruled grating (M, m=2), and the diffraction efficiency of the new grating has a flat character. Furthermore, the new grating reveals higher diffraction efficiency than the mechanically ruled grating, in an energy region over 200 eV.

Soft x-ray radiation with energies from a few tens to a few thousands electron volts (eV) is very useful to investigate atomic arrangements and electronic states in materials. Especially the radiation having energies ranging from several hundreds to about 3 keV is most suited to explore materials very familiar and important to human beings, for example, carbon, aluminum, and silicon. Recently, new viable soft x-ray sources such as synchrotron radiation and laser generated plasma x-rays have been developed. However we have had no good instruments so far to extract efficiently monochromatic light having an energy of a few keV. JAERI has progressed the development of efficient, high resolution grating spectrometers. Usually, grating grooves are fabricated on an optical glass by the mechanical ruling method or by the method utilizing optical interference. By the optical method, however, it has been impossible to make a grating for a soft x-ray spectrograph having a flat image plane. We have therefore developed a new optical method to generate interference fringes adequate for a flat-field spectrograph (Fig. 4-10). It is found that the new grating has a high diffraction efficiency compared to a mechanically ruled grating especially in a high energy region (Fig. 4-11). The new recording method of interference fringes developed here can be easily extrapolated to the fabrication of gratings used in a multi-keV region, and it is expected that the diffraction gratings fabricated by the new method will play a crucial role in the research of special materials.

Reference M. Koike et al., Design of Holographic Gratings Recorded with Aspheric Wave-Front Recording Optics for Soft x-ray Flat-Field Spectrographs, J. Electron Spectrosc. Relat. Phenom., 101-103, 913 (1999).

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