Ellipsoidal mirrors are ideal focusing optics for soft x-rays because of advantages that include high numerical aperture, high efficiency, and no chromatic aberrations. Shape accuracy of nanometer order is required on the internal surface of a mirror with a diameter of around 10 mm. Because of the difficulty of processing the internal surface, ellipsoidal mirrors are fabricated by replication of the shapes of master mandrels. In previous studies, a fabrication process was developed for x-ray ellipsoidal mirrors involving mandrel fabrication and nickel electroforming. 40-mm-long ellipsoidal mirrors were fabricated and a focused beam with full width at half maximum (FWHM) of 240 nm was obtained. For better focusing performance and expansion of the applicable energy range, we designed and fabricated a 120-mm-long ellipsoidal mirror from the master mandrel with a shape accuracy of 3.8 nm (root mean square). A focusing experiment was also performed at the synchrotron radiation facility, SPring-8 (BL25SU). A focused beam with FWHM of 1 μm was obtained.
An ellipsoidal mirror is a soft X-ray reflective focusing device. We are developing precise ellipsoidal mirrors based on an electroforming process. To improve the fabrication process, three-dimensional shape measurements with a high accuracy are required. In this research we develop a method to measure ellipsoidal shapes by industrial X-ray computed tomography (CT). The X-ray CT process consists of measuring the mirror shape and determining the parameters of the ellipsoid. We also evaluate the reproducibility of X-ray CT measurements and clarify that the accuracy is at the 5-m level.
In synchrotron radiation facilities, soft X-ray nanofocusing with mirrors remains a hurdle due to difficulties in mirror fabrication. We have been researching the use of ellipsoidal mirrors for soft X-ray nanofocusing. Information on the wavefront errors of focused beams is helpful for improving the focusing system. This study presents ptychographic wavefront measurements for a nanofocusing system with an ellipsoidal mirror. We developed a ptychography program and performed several simulations at 300 eV to investigate the theoretical accuracy of the wavefront measurements. The simulation results indicate that wavefront measurements with high accuracy are possible.
The Wolter mirror is a promising imaging device for soft x-ray microscopy owing to its excellent characteristics. Its annular aperture enables high-NA design while maintaining high photon transfer efficiency. However, its deep and narrow cylinder-like shape makes its fabrication difficult. Despite its long history, the Wolter mirror has not been practically used for high-resolution microscopy. We have been developing a fabrication process for grazing incidence mirrors with rotationally symmetric shapes. The mirrors are replicated from precisely machined mandrels. We employ electroforming as a replication method with high replication accuracy and reproductivity. Here, we report the first fabrication of a Wolter mirror and discuss the replication quality in electroforming. The imaging quality of Wolter mirror is also evaluated in an observation experiment using a visible-light microscope.
For vortex beams, characterization and optimization of the optical system are important. However, wavefront measurements on focused vortex beams are difficult because they have complex phase and intensity distributions. As a measurement method, we proposed the use of ptychography, in which the intensity and phase of the beams are retrieved using several far-field diffraction patterns. We constructed an optical system with a He-Ne laser light source to clarify the usefulness of ptychography. Test vortex beams were produced by a spatial light modulator (SLM) and focused by a plano-convex lens. A pinhole was scanned on the focal plane for collection of the diffraction intensity profiles. The phase and intensity of the vortex beams on the focal plane were retrieved so that the calculated beams were consistent with the intensity data. The retrieved intensity and phase distributions were compared with distributions predicted using the inputs for the SLM. They agreed well, indicating that the ptychographic phase retrieval method can be used for precise characterization of vortex beams. This method is valuable for improving the performance of applications using vortex beams.
Focusing and imaging optics can be characterized by evaluating the wavefront error of the focused beam. We have bean developing a ptychographic phase retrieval method using a visible laser to measure the wavefront error. In this study, the measurement accuracy of the method is increased by improving both the phase retrieval algorithm and the experimental setup. The system is applied to the characterization of an ellipsoidal mirror used for the focusing of soft X-rays. The posture of the mirror can be measured with a resolution of 1.4 μrad. The wavefront error originating from the surface profile error can be detected with an accuracy of 0.01λ (root mean square).
Mirrors are key devices for creating various systems in optics. Focusing X-ray and extreme ultraviolet (EUV) light requires mirror surfaces with an extremely high accuracy. The figure of an ellipsoidal mirror is obtained by rotating an elliptical profile, and using such a mirror, soft X-ray and EUV light can be focused to dimensions on the order of nanometers without chromatic aberration. Although the theoretical performance of ellipsoidal mirrors is extremely high, the fabrication of an ideal ellipsoidal mirror remains problematic. Based on this background, we have been working to develop a fabrication system for ellipsoidal mirrors. In this proceeding, we briefly introduce the fabrication process and the soft X-ray focusing performance of the ellipsoidal mirror fabricated using the proposed process.
It is possible to achieve soft X-ray nanofocusing with a high efficiency and no chromatic aberration by using an ultraprecise ellipsoidal mirror. Surface figure metrology is key in the improvement of surface figure accuracy. In this study, we propose a ptychographic phase retrieval method using a visible light laser to measure the surface figure error profile of an ellipsoidal mirror. We introduce a simple experimental system for ptychographic phase retrieval and demonstrate the basic performance of the proposed system. Obtainable wavefront information provides both the figure error and the alignment of the ellipsoidal mirror that yield the best focusing. This developed method is required for offline adjustments when an ellipsoidal mirror is installed in the beamline of synchrotron radiation or X-ray free-electron laser light sources.