A Highly Efficient Nanofocusing System For Soft X Rays

The focusing of x rays is essential in various types of x-ray microscopy because the size and flux of the illuminating beam determine the spatial resolution and signal intensity, respectively. A system with grazing incidence mirrors allows highly efficient x ray focusing with beam dimensions on the nanometer scale. Tube-shaped mirrors with a large numerical aperture and high demagnification, including ellipsoidal mirrors and Wolter mirrors, have recently been developed for soft x-ray nanofocusing. However, these mirrors have low efficiency because they cannot focus the central part of the illumination beam. Here, we propose a two-stage focusing system, in which a ring-shaped beam formed by a ring-focusing mirror illuminates a quasi-Wolter mirror. A prototype of the system is experimentally constructed at beamline BL25SU-A of SPring-8. Observation of the wavefront error based on ptychography enables efficient alignment of the two mirrors. Using this alignment procedure, we achieve a spot size of less than 200nm at 300eV.