Achieving ultrahigh resolution with high efficiency: Optical design of the two-dimensional Resonant Inelastic X-ray Scattering (2D-RIXS) spectrometer at NanoTerasu beamline 02U

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A state-of-the-art Resonant Inelastic X-ray Scattering (RIXS) facility, composed of a dedicated beamline and a two-dimensional RIXS (2D-RIXS) spectrometer, has been constructed and commissioned at beamline 02U in NanoTerasu, Japan. This paper reports the optical design and optimization of this spectrometer, aiming for an ultrahigh energy resolution of <10 meV in the soft x-ray range. Conventional RIXS spectrometers using monochromatic incident x rays suffer from a severe trade-off between energy resolution and measurement efficiency, which makes achieving resolutions of <10 meV practically unfeasible. To overcome this limitation, we adopted the hν2 concept using dispersive incident x rays, based on a comparative study. This approach decouples the energy resolution from the incident bandwidth. We estimate that our 2D-RIXS spectrometer improves efficiency by more than a factor of 10 compared to conventional spectrometers. The 2D-RIXS spectrometer has been successfully commissioned, demonstrating that sub-10 meV resolution measurement is achievable with practical throughput. While development continues toward the ultimate design resolution, the successful implementation validates the design strategy and offers a pathway for future high-performance RIXS instrumentation.