Refractive X-ray Lenses Research Articles

Diamond planar refractive lenses for third- and fourth-generation X-ray sources.

The fabrication and testing of planar refractive hard X-ray lenses made from bulk CVD diamond substrates is reported. The lens structures were generated by electron-beam lithography and transferred by reactive-ion etching into the diamond. Various lens designs were fabricated and tested at 12.4 and 17.5 keV photon energy. Efficiencies of up to 71% and gains of up to 26 were achieved. A line focus of 3.2 micro m (FWHM) was measured. These lenses should be able to withstand the extreme flux densities expected at the planned fourth-generation X-ray sources.

Nanofocusing parabolic refractive x-ray lenses

Parabolic refractive x-ray lenses with short focal distance can generate intensive hard x-ray microbeams with lateral extensions in the 100 nm range even at a short distance from a synchrotron radiation source. We have fabricated planar parabolic lenses made of silicon that have a focal distance in the range of a few millimeters at hard x-ray energies. In a crossed geometry, two lenses were used to generate a microbeam with a lateral size of 380 nm by 210 nm at 25 keV in a distance of 42 m from the synchrotron radiation source. Using diamond as the lens material, microbeams with a lateral size down to 20 nm and below are conceivable in the energy range from 10 to 100 keV.

Deep reactive ion etching of silicon and diamond for the fabrication of planar refractive hard X-ray lenses

We report on the fabrication of diamond and silicon refractive X-ray lenses. These are made using e-beam lithography and deep reactive ion etching. The diamond lenses are promising candidates for optics meeting the extreme requirements of planned X-ray sources based on the free electron laser principle. The silicon lenses are well suited for micro-focusing applications at hard X-ray energies above 20 keV. We measured efficiencies up to 78% and a width of the line-focus down to 3 μm for these lenses. The possibility to compensate for lens aberrations using stacked lenses is discussed. A simple method for the stacking of two silicon devices (lenses) by means of micro-mechanical alignment marks is shown, giving an alignment accuracy in the micron range.

On the theory of X-ray refractive optics: Exact solution for a parabolic medium

The exact solution is obtained for a propagator describing x-ray propagation through a refractive parabolic medium. Such a medium arises in compound many-element refractive x-ray lenses that are used in synchrotron radiation sources. The solution obtained allows one to analyze such lenses in detail to predict their operation in particular applications (beam focusing, microobject imaging, and Fourier transform).

Multi-prism x-ray lens

Refractive x-ray lenses with a triangular surface profile have been used to focus a synchrotron beam to sub-μm line width. These lenses are free from spherical aberration and work in analogy with one-dimensional focusing parabolic compound refractive lenses. However, the focal length can be easily varied by changing the gap between the two jaws. Silicon lenses were fabricated by wet anisotropic etching, and epoxy replicas were molded from the silicon masters. The lenses provided intensity gains up to a factor of 32 and the smallest focal line width was 0.87 μm. The simplified geometry and associated fabrication technique open possibilities for low-Z materials such as beryllium, which should greatly enhance the performance of refractive x-ray optics.

Nanotomography based on hard x-ray microscopy with refractive lenses

Based on parabolic refractive x-ray lenses we have built a hard x-ray microscope that allows one to image the interior of opaque samples with submicrometer resolution. We have combined magnified imaging with tomography to obtain the three-dimensional structure of the sample at a resolution well below 1 μm. Using an aluminum lens to record a magnified tomogram of a test sample (microprocessor), a resolution of slightly above 400 nm was found for the three-dimensional reconstruction. Lenses made of beryllium are expected to improve this resolution to well below 100 nm. The resulting challenges concerning instrumentation and numerical methods are discussed.

Parabolic refractive X-ray lenses for microscopy and microanalysis

Parabolic refractive X-ray lenses for microscopy and microanalysis

Deep lithography with a low energy synchrotron source: fabrication of X-ray refractive single lenses

Lithography as deep as 400 μm has been carried out to fabricate X-rays refractive lenses using a low energy synchrotron source (AURORA-2 S, 0.7 GeV). The lens made of PMMA has two parabolic curvatures with radii R=4 μm and apertures A=2(2Rz)1/2=179 μm, thus the aspect ratio z/R=250 for its curvatures, which is too great for traditional techniques to achieve. Upon fabrication of the lenses, precision of the curvatures has been evaluated by digital imaging analysis. The lens can singly focus a beam of hard X-rays into several microns at a reasonable focal length F=1.5 m. Advantages of using a low energy source for the LIGA process will be discussed regarding problems such as thick absorbers demanded by the LIGA mask and heat-load occurring in thick resist layers.

Parabolic refractive X-ray lenses.

Parabolic refractive X-ray lenses are optical components, especially suitable for third-generation synchrotron radiation sources. This article describes the status of the development of our lenses and illustrates the possibilities for micrometre and submicrometre focusing and for X-ray imaging in absorption and phase contrast. The parabolic lens profile ensures distortion-free imaging of high quality. First characteristics of Be lenses are given. A microscope based on Be lenses is expected to have a lateral resolution below 80 nm.

Refractive optics using lithium metal

Thanks to its low x-ray absorption, lithium should be the material of choice for x-ray refractive lenses. This article discusses some of the measurements done to verify lithium’s relevant properties. Both x-ray transmission and refraction are consistent with expectations. The lens gain suffers from broadening that is related to small-angle scattering.

Parabolic refractive X-ray lenses: a breakthrough in X-ray optics

Refractive X-ray lenses, considered for a long time as unfeasible, have been realized with a rotational parabolic profile at our institute: The main features of the new lenses are: they focus in two directions and are free of spherical aberration. By varying the number of individual lenses in the stack the focal length can be chosen in a typical range from 0.5 to 2 m for photon energies between about 6 and 60 keV . The aperture of the lens is about 1 mm matching the angular divergence of undulator beams at 3d generation synchrotron radiation sources. They cope without problems with the heat load from the white beam of an undulator. Finally, they are easy to align and to operate. Refractive X-ray lenses can be used with hard X-rays in the same way as glass lenses can be used for visible light, if it is take into account that the numerical aperture is small (of the order 10 −4). Being high-quality optical elements, the refractive X-ray lenses can be used for generating a focal spot in the μm range with a gain of a factor 1000 and more, or for imaging purposes as in a hard X-ray microscope. Recent examples from microanalysis, microtomography, fluorescence tomography, X-ray microscopy will be shown to demonstrate the state of the art. Possible new developments will be discussed.

High spatial resolution hard X-ray microscope using X-ray refractive lens and phase contrast imaging experiments

A high spatial resolution X-ray microscope was constructed using an X-ray refractive lens as an objective. The spatial resolution was tested using 18 keV X-ray. A 0.4 μm line and 0.4 μm space tantalum test pattern was successfully resolved. Using the similar setup with the addition of a phase plate, a Zernike type phase-contrast microscopy experiment was carried out for the phase retrieval of the samples. Two-dimensional phase-contrast images were successfully taken for the first time in the hard X-ray region. Images of a gold mesh sample were analyzed and the validity of this method was indicated. An improvement of the lens, however, is required for the precise phase retrieval of the samples.

Refractive X-ray lens for high pressure experiments at SPring-8

A stacked compound refractive X-ray lens was designed to produce an efficiently focused ( ϕ<0.1 mm) beam for high-pressure experiments at SPring-8. High-pressure X-ray diffraction requires an intense, high-energy and monochromatic X-ray beam in order to penetrate the absorptive window of a diamond anvil cell (DAC). Our lens, producing a focal spot of 120×275 μm 2 and a peak gain of 12, is well matched to these requirements. It is composed of many plastic chips made by molding, which allows many identical chips to be made precisely. Other advantages of this lens include high throughput, simple energy tunability and easy installation.

X-ray refractive planar lens with minimized absorption

Silicon refractive planar parabolic lenses with minimized absorption were fabricated by a combination of photolithography and dry-etching techniques. Focusing and spectral properties of the lenses were studied with synchrotron radiation in the energy range 8–25 keV at the European Synchrotron Radiation Facility. A focal spot of 1.8 μm with a gain of 18.5 and transmission of more then 80% was measured at 15.6 keV. The spectral characteristics were analyzed taking into account material dispersion and photon-energy attenuation in the hard x-ray range.

Glass capillary X-ray lens: fabrication technique and ray tracing calculations

A new method for producing a compound refractive X-ray lens is described. The lens is designed as glass capillary filled by a set of concave individual lenses. The method is appropriate for the preparation of 10–1000 spherical lenses in a glass capillary with a diameter of 0.1–1 mm. Lens aberrations are considered and simple formulas for spot size are derived. Ray-tracing simulations on the focusing experiments using 18 keV X-rays are reported.

Imaging by parabolic refractive lenses in the hard X-ray range

The manufacture and properties of compound refractive lenses (CRLs) for hard X-rays with parabolic profile are described. These novel lenses can be used up to ∼60 keV. A typical focal length is 1 m. They have a geometrical aperture of 1 mm and are best adapted to undulator beams at synchrotron radiation sources. The transmission ranges from a few % in aluminium CRLs up to about 30% expected in beryllium CRLs. The gain (ratio of the intensity in the focal spot relative to the intensity behind a pinhole of equal size) is larger than 100 for aluminium and larger than 1000 for beryllium CRLs. Due to their parabolic profile they are free of spherical aberration and are genuine imaging devices. The theory for imaging an X-ray source and an object illuminated by it has been developed, including the effects of attenuation (photoabsorption and Compton scattering) and of the roughness at the lens surface. Excellent agreement between theory and experiment has been found. With aluminium CRLs a lateral resolution in imaging of 0.3 µm has been achieved and a resolution below 0.1 µm can be expected for beryllium CRLs. The main fields of application of the refractive X-ray lenses are (i) microanalysis with a beam in the micrometre range for diffraction, fluorescence, absorption, scattering; (ii) imaging in absorption and phase contrast of opaque objects which cannot tolerate sample preparation; (iii) coherent X-ray scattering.

X-ray focusing test and x-ray imaging test by a microcapillary x-ray lens at an undulator beamline

A first focusing test of the undulator radiation at SPring-8 has been done using two types of x-ray refractive lenses in thin glass capillaries. One (lens No. 1) contained bubbles in a glue whereas the other (lens No. 2) contained bubbles in glycerol. The capillaries had inner diameters of 0.2 and 0.8 mm, respectively. Using x rays of 17–18 keV, the undulator source image was investigated at the focal plane. The spherical aberrations and the field distortions were carefully examined by taking the images of a gold mesh. Lens No. 1 had an advantage of high transmissivity in the hard x-ray region (18% at 18 keV) and high tolerance to severe radiation damage, e.g., ∼5×1012 photons/s/0.03 mm2 of the 18 keV x rays for an exposure time of 1 h. On the other hand, lens No. 2 had an advantage of a large aperture, 0.8 mm, and a small field distortion, e.g., less than 10% inside a diameter of 300 μm.

Cylindrical compound refractive x-ray lenses using plastic substrates

We have measured the intensity profile of x rays focused by compound refractive lenses (CRLs) fabricated using acrylic (Lucite) and polyethylene plastics. A linear array of closely spaced holes acts as multiple cylindrical lenses. The important parameters for this type of focusing are the focal length and absorption, and, for wavelengths shorter than 3 Å, low atomic number plastics are suitable. We have experimentally demonstrated that we can achieve one-dimensional focusing for photon energies between 9 and 19.5 keV with focal lengths between 20 and 100 cm. For example, using 12 keV x rays we have achieved focal full width at half maximum linewidths down to 21 μm at a distance of only 20 cm from the CRL. The x-ray source was a synchrotron emitter whose source size in the vertical dimension was 445 μm.

A microscope for hard x rays based on parabolic compound refractive lenses

We describe refractive x-ray lenses with a parabolic profile that are genuine imaging devices, similar to glass lenses for visible light. They open considerable possibilities in x-ray microscopy, tomography, microanalysis, and coherent scattering. Based on these lenses a microscope for hard x rays is described, that can operate in the range from 2 to 50 keV, allowing for magnifications up to 50. At present, it is possible to image an area of about 300 μm in diameter with a resolving power of 0.3 μm that can be increased to 0.1 μm. This microscope is especially suited for opaque samples, up to 1 cm in thickness, which do not tolerate sample preparation, like many biological and soil specimens.

High resolution X-ray imaging using a silicon strip detector

The authors present the first images and an initial evaluation of a scanned-slot X-ray imaging system based on edge-on silicon strip detectors and high-speed low-noise parallel processing ASICs. The authors have demonstrated noiseless single photon counting above a minimal threshold of 7.2 keV. Edge scans show negligible cross talk between different channels in the ASIC. The Modulation Transfer Function (MTF) has been measured and found to agree with the ideal MTF for 100 /spl mu/m pixel size. The first images are obtained at very low exposures and show the high performance of the system. The authors also present a way of enhancing the X-ray flux to a slot by using a refractive X-ray lens. They believe this focusing device will significantly enhance the potential for scanned-slot X-ray imaging.