In X-ray focusing, grazing incidence mirrors offer advantages of no chromatic aberration and high focusing efficiency. Although nanofocusing mirrors have been developed for the hard X-ray region, there is no mirror with nanofocusing performance in the soft X-ray region. Designing a system with the ability to focus to a beam size smaller than 100 nm at an X-ray energy of less than 1 keV requires a numerical aperture larger than 0.01. This leads to difficulties in the fabrication of a soft X-ray focusing mirror with high accuracy. Ellipsoidal mirrors enable soft X-ray focusing with a high numerical aperture. In this study, we report a production process for ellipsoidal mirrors involving mandrel fabrication and replication processes. The fabricated ellipsoidal mirror was assessed under partial illumination conditions at the soft X-ray beamline (BL25SU) of SPring-8. A focal spot size of less than 250 nm was confirmed at 300 eV. The focusing tests indicated that the proposed fabrication process is promising for X-ray mirrors that have the form of a solid of revolution, including Wolter mirrors.

Intense sub‐micrometre focusing of a soft X‐ray free‐electron laser (FEL) was achieved by using an ellipsoidal mirror with a high numerical aperture. A hybrid focusing system in combination with a Kirkpatrick–Baez mirror was applied for compensation of a small spatial acceptance of the ellipsoidal mirror. With this system, the soft X‐ray FEL pulses were focused down to 480 nm × 680 nm with an extremely high intensity of 8.8×1016 W cm−2 at a photon energy of 120 eV, which yielded saturable absorption at the L‐edge of Si (99.8 eV) with a drastic increase of transmittance from 8% to 48%.

An x-ray ellipsoidal mirror is an ideal tool for focusing soft x-rays. Because nanometer-level shape accuracy is required in the internal surface of a mirror having a small diameter, it is difficult to fabricate the mirror by processing the surface directly. We developed a fabrication process for soft x-ray ellipsoidal mirrors in which the surface of a high-precision quartz mandrel with the inverted shape of the designed mirror is replicated by nickel sulfamate electroforming. In this study, an ellipsoidal mirror of 40-mm length was fabricated and the shape accuracy of the replicated surface was evaluated by a measurement method using a contact probe. The root mean square (RMS) of the replication error in the entire measured surface was 27.2 nm. When the evaluated area was half the replicated surface near the middle of the mirror, the RMS of the replication error was 14.7 nm. Wave-optical simulation suggested that it is possible to focus soft x-rays to a spot with a diameter of 400 nm.

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A soft X-ray ptychography system using a Wolter mirror for the illumination optics has been developed. By taking advantage of the achromaticity of the optics, the system is capable of seamlessly imaging at half-period resolution of 50 nm with a broad photon-energy range from 250 eV to 2 keV while maintaining the focal position. Imaging a mammalian cell at various wavelengths was demonstrated, and high-resolution visualization of organelle was achieved. Stereo imaging was also performed with a long working distance of 20 mm. In combination with in-situ/operando and tomographic measurements, this system will be a powerful tool for observing biological and material targets with complex features.