Appochromatic X-ray focusing

Diffractive lenses and refractive lenses are widely used in X-ray analysis and high-resolution X-ray microscopy systems. However, the high dispersion characteristics of these two lenses lead to different X-ray focal positions of different wavelengths, resulting in chromatic aberration during imaging and greatly reducing imaging quality. Therefore, X-ray microscopy imaging systems using diffractive or refractive lenses often use highly monochromatic light to avoid chromatic aberrations, at the cost of a large amount of X-rays being wasted.

In the field of visible light, achromatic lenses have been used for more than 100 years, through two glasses with different dispersion forces and double lenses that meet the corresponding curvature conditions. In the field of X-rays, the difference in dispersion force of most substances to X-rays is very small, and the same method is not applicable.

At the beginning of the 21st century, researchers proposed a new type of solution that combines refractive lenses and diffractive lenses to achieve achromatic conditions by using the significant difference in dispersion force between the two types of lenses. However, due to the limitations of the level of manufacturing technology, the design of such solutions is limited to the theoretical stage.

In recent years, the rapid development of micro-nano manufacturing technology, and the maturity of 3D printing technology based on two-photon polymerization, has made it possible to manufacture composite refractive lenses with high numerical apertures suitable for this system.

Recently, researchers such as Umut T. Sanli and Qi Peng of the Paul Schell Institute in Switzerland, Griffin Rodgers of the University of Basel and Jan Garrevoet of the German Electron Synchrotron Institute (DESY) published a research paper in Light: Science & Applications entitled “Apochromatic X-ray focusing”.

Achromatic lenses consist of two lenses with different dispersion capabilities, which can focus two different wavelengths of light to the same point, so as to achieve achromatic effects in a certain wavelength range.

The apochromatic lens can be considered an improved version of the achromatic lens, and the chromatic shift curve is a cubic equation that focuses three different wavelengths of light to one point, increasing the wavelength range of achromatic by several times, see Figure 1.

Figure 1: X-ray apochromatic focusing principle: the refractive lens and Fresnel band sheet are placed back and forth at specific intervals, the chromatic aberration corrects for each other, and three different energies/wavelengths of X-rays can be focused at point F at the same time.

In the field of visible light, achromatic and apochromatic lenses have existed for more than a hundred years. In the field of X-rays, it was not until 2022 that the world’s first achromatic lens was introduced. This paper reports the world’s first X-ray apochromatic lens system successfully developed by the research team using Fresnel band sheet (FZP) and composite refractive lens (CRL) that meet special conditions based on the work of achromatic lenses. Experiments show that the apochromatic lens exhibits good achromatic effect in the energy range of 7 keV to 12 keV, and the achromatic range is increased by four times compared with the achromatic lens, which can be more widely used in chromatic aberration correction of refractive and diffractive lenses.

The apochromatic X-ray lens system developed by this study consists of two independent optical elements: a composite refractive lens manufactured by two-photon polymerization 3D printing technology, and a Fresnel bandpiece manufactured by electron beam lithography and gold plating, as shown in Figure 2.

Figure 2. Components of an X-ray apochromatic lens. a) 3D printed optical microscope image of divergent CRL placed on a 250 nm thick silicon nitride film; b) Scanning electron microscope image of a composite refractive lens and c) a 45-degree viewing angle of view of a waveband sheet; d) Composite refractive lens (bottom left) versus matchstick.

X-ray scanning transmission microscopy and tandemography measurements on the PETRA III synchrotron P06 beamline in Germany showed that the lens system exhibited excellent achromatic performance over an X-ray energy range of 7 to 12 keV, as shown in Figure 3.

Figure 3: Scanning transmission microscopy of the Siemens star test sample in different energy X-ray beams (no change in position on the optical axis). Two different FZP-CRL separation distances d are shown.

Compared with the original X-ray achromatic lens reported above, the effective energy range of the apochromatic lens is increased by a factor of four. The implementation of this system is of great significance for time-resolved experiments with short exposure time and high signal-to-noise ratio.

The submicron-sized focus of the system resolves test samples with a linewidth of 480 nm. The development of nano-3D printing technology makes the preparation of optical elements with higher numerical apertures not difficult, and its spatial resolution can be further improved. However, due to the absorption of X-rays by refractive lenses, achieving high resolutions in the 100 nm range remains challenging. In the hard X-ray band, relatively low X-ray absorption can lead to greater room for improvement. The rapid development of nanoscale 3D printing technology will be the key to achieving the manufacturing of the desired refractive structure.

The advent of X-ray achromatic and apochromatic lenses is a landmark advance in the field of X-ray microscopy and may even replace existing mirror systems with their economical, compact and coaxial imaging advantages, and will play an increasingly important role in accelerator and laboratory X-ray source-based microscopic imaging systems. (Source: China Optics)

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