Diffraction gratings with two-orders-of-magnitude-enhanced dispersion rates for sub-meV-resolution soft x-ray spectroscopy

Diffraction gratings with large angular dispersion rates are central to obtaining high spectral resolution in grating spectrometers operating over a broad spectral range from infrared to soft-x-ray domains. The greatest challenge is of course to achieve large dispersion rates in the short-wavelength x-ray domain. Here we show that crystals in non-coplanar asymmetric x-ray Bragg diffraction can function as high-reflectance soft-x-ray diffraction gratings with dispersion rates that are at least two orders of magnitude larger than those that are possible with state-of-the-art man-made gratings. This opens new opportunities to design and implement soft x-ray resonant inelastic scattering (RIXS) spectrometers with spectral resolutions that are up to two orders of magnitude higher than what is currently possible, to further advance a very dynamic field of RIXS spectroscopy, and to make it competitive with inelastic neutron scattering. We present examples of large-dispersion-rate crystal diffraction gratings operating near the 930-eV L$_3$ absorption edge in Cu and of the 2.838-keV L$_3$ edge in Ru.