Achieving Ultrahigh Resolution with High Efficiency: Optical Design of the 2D-RIXS Spectrometer at NanoTerasu BL02U

A state-of-the-art Resonant Inelastic X-ray Scattering (RIXS) facility, composed of a dedicated beamline and a 2D-RIXS spectrometer, has been constructed and commissioned at BL02U 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 $hν^2$ 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 towards the ultimate design resolution, the successful implementation validates the design strategy and offers a pathway for future high-performance RIXS instrumentation.