X-ray ring-focusing mirror

Hidekazu Mimura*, Yoko Takeo, Hiroto Motoyama, Yasunori Senba, Hikaru Kishimoto, Haruhiko Ohashi 

Mirrors used in synchrotron radiation facilities can reflect X-ray beams ideally owing to recent advancements in ultraprecise fabrication technologies. Flat mirrors can change the direction of an X-ray beam without distorting its wavefront. Elliptically figured mirrors can focus X-rays onto a spot with nanometer dimensions. The intensity distribution of a reflected X-ray beam is determined, and thus can be controlled, by the shape of the employed mirror. This study proposes, fabricates, and evaluates a ring-focusing mirror that can produce an X-ray beam with a ring-shaped intensity profile. This ring-shaped X-ray beam could be created and observed at the soft X-ray beamline BL25SU at the SPring-8 synchrotron radiation facility, Japan. The increase in the freedom of X-ray mirror shapes will lead to the development of various analytical methods and optical systems in the X-ray region.

Ultraprecise mirrors are indispensable for steering X-ray beams at beamlines at synchrotron radiation and X-ray free electron laser facilities. Thanks to the advancements in precision machining and measurement technologies, mirrors have high accuracy, reflecting X-ray beams without distorting their wavefront and without X-ray scattering at reflections.
Such X-ray mirrors are fabricated using a deterministic figure correction system that utilizes machining and measurement methods. In this system, differences between the measured profiles and the designed profile are decreased by controlling the scanning speed distribution in the stages of a processing machine. Ion beam figuring and elastic emission machining are used for the figure correction of X-ray mirrors. A long trace profiler and stitching interferometry are used as measurement methods. A peak-to-valley figure accuracy of 2 nm can be achieved for mirror surfaces, even for mirror lengths above 400 mm.
Focus widths of 25 and 7 nm have been achieved for one-dimensional hard X-ray focusing using a total reflection mirror and a multilayer mirror, respectively. An X-ray free electron laser can be focused to a size of 50 nm. Recently, ellipsoidal mirrors that can focus soft and hard X-rays onto a few hundred nanometer spot using one reflection have been developed. These focusing mirrors have shapes designed using elliptical or ellipsoidal functions with two focal points, where one focal point is the source and the other focal point is the collection point of X-rays.
The intensity and phase distributions of a reflected beam are determined, and thus can be controlled, by the figure of the mirror. We have previously conducted theoretical studies on X-ray mirrors that can produce ring-shaped intensity distributions on the focal plane. We call this type of mirror an X-ray ring-focusing mirror. An X-ray beam with a spherical wavefront is reflected on a surface with a specific shape and turned into an X-ray beam whose intensity distribution is a circle. After the focal plane, the X-ray beam diverges, with no intensity at the center.
A theoretical study suggested that a two-stage soft X-ray focusing system that utilizes a ring-focusing mirror and a quasi-ellipsoidal mirror enables soft X-ray focusing to a size of less than 10 nm without chromatic aberration or flux loss, making it suitable for coherent X-rays produced by X-ray-free electron lasers. A procedure was developed for determining the shape of the X-ray ring-focusing mirrors, and several geometric and wave optic simulations were used to verify the design procedure. For X-ray focusing with a zone plate, it is necessary to block the X-rays in the center region. X-ray ring-focusing mirrors are suitable for producing an illumination beam for zone plates. Ring-focusing mirrors can also be used with hard X-rays.
In this letter, a prototype ring-focusing mirror was fabricated and tested at soft X-ray beamline BL25SU at the SPring-8 synchrotron radiation facility, Japan.