Nanofocusing Mirrors Research Articles

Fabrication of a precise ellipsoidal mirror for soft X-ray nanofocusing.

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.

Compensation of X-ray mirror shape-errors using refractive optics

Focusing of X-rays to nanometre scale focal spots requires high precision X-ray optics. For nano-focusing mirrors, height errors in the mirror surface retard or advance the X-ray wavefront and after propagation to the focal plane, this distortion of the wavefront causes blurring of the focus resulting in a limit on the spatial resolution. We describe here the implementation of a method for correcting the wavefront that is applied before a focusing mirror using custom-designed refracting structures which locally cancel out the wavefront distortion from the mirror. We demonstrate in measurements on a synchrotron radiation beamline a reduction in the size of the focal spot of a characterized test mirror by a factor of greater than 10 times. This technique could be used to correct existing synchrotron beamline focusing and nanofocusing optics providing a highly stable wavefront with low distortion for obtaining smaller focus sizes. This method could also correct multilayer or focusing crystal optics allowing larger numerical apertures to be used in order to reduce the diffraction limited focal spot size.

At-wavelength metrology of hard X-ray mirror using near field speckle

We present a method to measure the surface profile of hard X-ray reflective optics with nanometer height accuracy and sub-millimetre lateral resolution. The technique uses X-ray near-field speckle, generated by a scattering membrane translated using a piezo motor, to infer the deflection of X-rays from the surface. The method provides a nano-radian order accuracy on the mirror slopes in both the tangential and sagittal directions. As a demonstration, a pair of focusing mirrors mounted in a Kirkpatrick-Baez (KB) configuration were characterized and the results were in good agreement with offline metrology data. It is hoped that the new technique will provide feedback to optic manufacturers to improve mirror fabrication and be useful for the online optimization of active, nano-focusing mirrors on modern synchrotron beamlines.

Stitching-angle measurable microscopic-interferometer: Surface-figure metrology tool for hard X-ray nanofocusing mirrors with large curvature

We propose an optical profilometer system for evaluating mirrors that are used for hard X-ray nanofocusing. A surface metrology system designed for these mirrors to focus to a size of 10 nm is required to ensure a precise measurement of a large curvature with a vertical accuracy of 1 nm and a lateral resolution less than 0.1 mm. The surface metrology system is based on a stitching interferometer that can determine stitching angles between adjacent local surface profiles. Local surface profiles and stitching angles are detected simultaneously by using two types of optical interferometers, a microscopic interferometer and a large-scale interferometer, respectively. We developed a nanofocusing mirror that has approximately the same surface curvature as a sub-10-nm focusing mirror, to examine the measurement accuracy of the developed surface metrology system. As a result of a performance assessment of the measurement instrument, we achieved measurement reproducibility and reliability of less than peak-to-valley height of 4 nm by measuring the focusing mirror. In addition, we achieved focusing performance of a full width at half maximum 27 nm at 7 keV using the mirror at SPring-8. We verified that the metrology system can be used to measure sub-10-nm focusing mirrors.

Stitching interferometric measurement system for hard x-ray nanofocusing mirrors

We have developed an advanced type of relative angle determinable stitching interferometry (RADSI) for hard x-ray nanofocusing mirrors. The surface metrology system consists of two types of optical interferometers. One is a microscope Michelson interferometer, which measures the local figure profile of the mirror surface, and the other is a large-area Fizeau interferometer, which determines the stitching angles between neighboring shots using a flat mirror. The surface figure profile of each of the one-shots and the stitching angles are measured at the same time. Precise measurement of local surface area and the stitching angles of 1×10-8 rad order were achieved using the developed system. To demonstrate the accuracy of the developed measurement system, we measured an elliptically figured mirror for hard x-ray nanofocusing. The mirror's surface shape has a curvature radius of a few meters. As a result, measurement reproducibility and reliability of less than PV 3 nm was achieved by using a 25 nm focusing mirror.