Abstract
A grand challenge in soft x-ray spectroscopy is to drive the resolving power of monochromators and spectrometers from the 10(4) achieved routinely today to well above 10(5). This need is driven mainly by the requirements of a new technique that is set to have enormous impact in condensed matter physics, Resonant Inelastic X-ray Scattering (RIXS). Unlike x-ray absorption spectroscopy, RIXS is not limited by an energy resolution dictated by the core-hole lifetime in the excitation process. Using much higher resolving power than used for normal x-ray absorption spectroscopy enables access to the energy scale of soft excitations in matter. These excitations such as magnons and phonons drive the collective phenomena seen in correlated electronic materials such as high temperature superconductors. RIXS opens a new path to study these excitations at a level of detail not formerly possible. However, as the process involves resonant excitation at an energy of around 1 keV, and the energy scale of the excitations one would like to see are at the meV level, to fully utilize the technique requires the development of monochromators and spectrometers with one to two orders of magnitude higher energy resolution than has been conventionally possible. Here we investigate the detailed diffraction characteristics of multilayer blazed gratings. These elements offer potentially revolutionary performance as the dispersive element in ultra-high resolution x-ray spectroscopy. In doing so, we have established a roadmap for the complete optimization of the grating design. Traditionally 1st order gratings are used in the soft x-ray region, but we show that as in the optical domain, one can work in very high spectral orders and thus dramatically improve resolution without significant loss in efficiency.
Highlights
The ability of blazed gratings to concentrate almost all of the diffracted energy into a high diffraction order makes them the element of choice when compared to conventional lamellar and sinusoidal gratings
Very high aspect ratio structures can be produced by Deep Reactive Ion Etching (DRIE) processes in multilayers [4], it is extremely challenging to meet the high quality required to provide high diffraction efficiency at high order operation of the structure
The most of the results shown in the paper were obtained for the TE polarization, which is relevant for our synchrotron applications
Summary
The ability of blazed gratings to concentrate almost all of the diffracted energy into a high diffraction order makes them the element of choice when compared to conventional lamellar and sinusoidal gratings. Very high aspect ratio structures can be produced by Deep Reactive Ion Etching (DRIE) processes in multilayers [4], it is extremely challenging to meet the high quality required to provide high diffraction efficiency at high order operation of the structure This is due to the fact that the thickness of the lamellae decrease with increasing order and for reasonable line densities become too thin to be practical in higher orders. Recent progress in fabrication of high quality saw-tooth substrates [13,14] by anisotropic etching of silicon single crystals [15,16] and optimization of the ML growth on such complicated substrates has enabled the demonstration of a diffraction efficiency of approximately 50% for ML-coated blazed gratings (MBG) in the EUV wavelength range [13,14,17,18,19].
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