Hard X-ray Nanofocusing Research Articles

Achieving hard X-ray nanofocusing using a wedged multilayer Laue lens.

We report on the fabrication and the characterization of a wedged multilayer Laue lens for x-ray nanofocusing. The lens was fabricated using a sputtering deposition technique, in which a specially designed mask was employed to introduce a thickness gradient in the lateral direction of the multilayer. X-ray characterization shows an efficiency of 27% and a focus size of 26 nm at 14.6 keV, in a good agreement with theoretical calculations. These results indicate that the desired wedging is achieved in the fabricated structure. We anticipate that continuous development on wedged MLLs will advance x-ray nanofocusing optics to new frontiers and enrich capabilities and opportunities for hard X-ray microscopy.

Hard X-ray nanofocusing using adaptive focusing optics based on piezoelectric deformable mirrors.

An adaptive Kirkpatrick-Baez mirror focusing optics based on piezoelectric deformable mirrors was constructed at SPring-8 and its focusing performance characteristics were demonstrated. By adjusting the voltages applied to the deformable mirrors, the shape errors (compared to a target elliptical shape) were finely corrected on the basis of the mirror shape determined using the pencil-beam method, which is a type of at-wavelength figure metrology in the X-ray region. The mirror shapes were controlled with a peak-to-valley height accuracy of 2.5 nm. A focused beam with an intensity profile having a full width at half maximum of 110 × 65 nm (V × H) was achieved at an X-ray energy of 10 keV.

Optimized short kinoform lenses for hard X-ray nano-focusing

An optimized structure of the short kinoform lens (KL) for hard X-ray nano-focusing is presented. By modifying the lower and upper surfaces of the conventional short KL, the focusing performance is improved largely. Unlike the conventional short KLs, whose effective numerical aperture (NA) is limited by the edge diffraction effects and multi-wave scattering, the optimized short KL is capable of achieving diffraction-limited focusing. The Takagi–Taupin description (TTD) of X-ray dynamical diffraction theory is employed to investigate the diffractive properties of the optimized short KL. When the harmonic number equals 1, the optimized short KL could be treated as a diffractive lens, whose effective NA could exceed the critical angle limit. According to their different focusing properties, the lens aperture could be divided into three regions, namely the refraction region, the transition region and the resonance region. As a comparison, with a large harmonic number, the optimized short KL behaves more like a refractive lens and its effective NA is restricted by the critical angle limit. By choosing appropriate harmonic number, maximum duty cycle and maximum propagation ratio, the difficulty in fabricating the proposed lens structure could be alleviated greatly without degrading the focusing performance obviously. This study indicates that, besides the arrangement of lens segments, their shapes are also important. The simulation results demonstrate that a short KL is also a potential candidate for hard X-ray nano-focusing.

Hard x-ray nanofocusing by refractive lenses of constant thickness

In order to focus light or x rays, the thickness of a refractive lens is typically varied over its aperture. Here, we present a refractive x-ray lens made of lamellae of constant thickness, the refractive lamellar lens. Refractive power is created by a specific bending of the lamellae rather than by a concave lens profile. This very special design has the technological advantage that materials like sapphire or diamond can be used to make lenses by coating techniques. A first lens prototype focused x rays with a photon energy E = 15.25 keV to a lateral beam size of 164 nm × 296 nm full width at half maximum.

Fabrication of laser deposited high-quality multilayer zone plates for hard X-ray nanofocusing

Abstract Recently, we demonstrated unprecedented sub-5 nm point focusing of hard X-rays (at 7.9 keV) based on the combination of a high gain Kirkpatrick–Baez (KB) mirror system and a high resolution W/Si multilayer zone plate (MZP). This MZP was prepared by the combination of pulsed laser deposition (PLD) and focused ion beam (FIB). Despite the small focus size, the MZP's quality suffered from sufficient but comparatively low efficiency (2%). In this paper we discuss how to overcome limitations of MZP fabrication by PLD by investigating the material systems W/Si, W/ZrO2, and Ta2O5/ZrO2. We give a detailed description on the optimization processes for the deposition of smooth multilayers with highly precise layer thicknesses on a rotating wire. Furthermore, we present our latest results regarding a Ta2O5/ZrO2 MZP, which has been proven already to be a system of high potential in the very first experiments as the efficiency reached 6.9% (at 18 keV).

Theoretical investigation of higher orders optimized Multilayer Laue Lens for hard x-ray nano-focusing

The higher orders optimized Multilayer Laue Lens (MLL) was proposed in this paper to further improve the focusing resolution of hard x-rays. Based on the dynamical diffraction theory, the higher orders diffraction efficiency can be greatly improved through optimizing the thickness ratio (γ=dabsorp/Λ, Λ is the period of local gratings) and cross-section depth. For an optimized WSi2/Si MLL with an outmost layer period of 16nm and γ=0.3–0.25, the average efficiency of the -2nd and -3rd order diffractions are increased to 55.5% and 49.3%, respectively. The efficiencies are almost the same as the -1st order diffraction (55.9%). The focusing property of the optimized MLL with only 40% of the full structure was simulated. Compared to the -1st order, the focal line width of higher orders is decreased by a factor of k (k is the diffraction order) which is 9.0nm and 5.9nm for the -2nd and -3rd order, respectively. It is demonstrated that the higher orders optimized MLL can be used to further improve the focusing resolution of hard x-rays with high efficiency without fabricating ultra-thin outmost structures.

Achromatic nested Kirkpatrick–Baez mirror optics for hard X-ray nanofocusing

The first test of nanoscale-focusing Kirkpatrick-Baez (KB) mirrors in the nested (or Montel) configuration used at a hard X-ray synchrotron beamline is reported. The two mirrors are both 40 mm long and coated with Pt to produce a focal length of 60 mm at 3 mrad incident angle, and collect up to a 120 µm by 120 µm incident X-ray beam with maximum angular acceptance of 2 mrad and a broad bandwidth of energies up to 30 keV. In an initial test a focal spot of about 150 nm in both horizontal and vertical directions was achieved with either polychromatic or monochromatic beam. The nested mirror geometry, with two mirrors mounted side-by-side and perpendicular to each other, is significantly more compact and provides higher demagnification than the traditional sequential KB mirror arrangement. Ultimately, nested mirrors can focus larger divergence to improve the diffraction limit of achromatic optics. A major challenge with the fabrication of the required mirrors is the need for near-perfect mirror surfaces near the edge of at least one of the mirrors. Special polishing procedures and surface profile coating were used to preserve the mirror surface quality at the reflecting edge. Further developments aimed at achieving diffraction-limited focusing below 50 nm are underway.

Multilayer Laue Lens: A Path Toward One Nanometer X-Ray Focusing

The multilayer Laue lens (MLL) is a novel diffractive optic for hard X-ray nanofocusing, which is fabricated by thin film deposition techniques and takes advantage of the dynamical diffraction effect to achieve a high numerical aperture and efficiency. It overcomes two difficulties encountered in diffractive optics fabrication for focusing hard X-rays: (1) small outmost zone width and (2) high aspect ratio. Here, we will give a review on types, modeling approaches, properties, fabrication, and characterization methods of MLL optics. We show that a full-wave dynamical diffraction theory has been developed to describe the dynamical diffraction property of the MLL and has been employed to design the optimal shapes for nanofocusing. We also show a 16 nm line focus obtained by a partial MLL and several characterization methods. Experimental results show a good agreement with the theoretical calculations. With the continuing development of MLL optics, we believe that an MLL-based hard x-ray microscope with true nanometer resolution is on the horizon.

Hard X-ray nano-focusing with Montel mirror optics

Kirkpatrick–Baez mirrors in the Montel (or nested) configuration were tested for hard X-ray nanoscale focusing at a third generation synchrotron beamline. In this scheme, two mirrors, mounted side-by-side and perpendicular to each other, provide for a more compact focusing system and a much higher demagnification and flux than the traditional sequential K–B mirror arrangement. They can accept up to a 120 μm×120 μm incident X-ray beam with a long working distance of 40 mm and broad-bandpass of energies up to ∼30 keV. Initial test demonstrated a focal spot of about 150 nm in both horizontal and vertical directions with either polychromatic or monochromatic beam. Montel mirror optics is important and very appealing for achromatic X-ray nanoscale focusing in conventional non-extra-long 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.

Mo/Si and MoSi2/Si nanostructures for multilayer Laue lens

To develop a multilayer Laue lens (MLL), we fabricated depth-graded Mo/Si and MoSi2/Si multilayers with each boundary according to the Fresnel zone configuration. The multilayers were deposited by magnetron sputtering. From the result of SEM image analysis of the multilayer cross sections, MoSi2/Si multilayer had smaller layer-thickness errors than Mo/Si multilayer. In addition, from the result of the focusing test by using 20-keV X-rays, the measured beam size of MoSi2/Si MLL had a small blurring from the diffraction limited beam size. These results suggest that MoSi2/Si multilayer is better suited than Mo/Si multilayer for use as an MLL in hard x-ray nanofocusing.

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.

Comparison of two methods for simulation of hard X-ray nanofocusing by elliptical mirrors

Wave-optical calculations are essential for predicting the X-ray focusing performance of precisely figured elliptical mirrors. The complex wavefield in the vicinity of the focal plane of a mirror with RMS height error in the nanometer range compared to the best-fit ellipse has been calculated using two methods. A pupil function method that treats the surface topography of a mirror as an aberration to a perfect ellipse was used to obtain the reflected amplitude and phase around the focal point downstream. The results were compared with direct propagation of waves from a point source, and it was found that both methods were in good agreement. Each approach provides advantages that are useful in designing mirrors to achieve diffraction limited focusing.

Relative angle determinable stitching interferometry for hard x-ray reflective optics

Metrology plays an important role in surface figuring with subnanometer accuracy. We have developed relative angle determinable stitching interferometry for the surface figuring of elliptical mirrors, in order to realize hard x-ray nanofocusing. In a stitching system, stitching angles are determined not by the general method using a common area between neighboring shots, but by the new method using the mirror’s tilt angles measured at times when profile data are acquired. The high measurement accuracy of approximately 4nm (peak-to-valley) was achieved in the measurement of a cylindrical surface having the same curvature as the elliptically designed shape to enable hard x-ray nanofocusing.