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

TL;DR

This paper presents the optical design of a 2D-RIXS spectrometer achieving ultrahigh energy resolution (<10 meV) with over tenfold efficiency improvement, enabling practical high-resolution measurements in soft X-ray spectroscopy.

Contribution

It introduces a novel optical design using the $h u^2$ concept with dispersive incident X-rays to decouple energy resolution from incident bandwidth, surpassing conventional trade-offs.

Findings

Achieved sub-10 meV energy resolution in soft X-ray RIXS.

Demonstrated over tenfold increase in measurement efficiency.

Validated the design strategy through successful commissioning.

Abstract

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 $h\nu^2$ 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.