Apochromatic X-ray focusing

Diffractive and refractive lenses are widely used in X-ray analysis and high-resolution X-ray microscopy with many scientific applications in materials science, energy sciences and biology. Nevertheless, both types of X-ray optics suffer from a strong chromatic aberration, meaning that they focus different X-ray energies/wavelengths at different distances along the optical axis. As a result, many high-resolution X-ray analysis techniques can only be implemented with monochromatic X-ray beams, meaning that in many cases a large portion of the X-ray intensity is sacrificed.

The apochromatic X-ray optic consisted of two independent optical elements separated by a distance: a divergent refractive X-ray lens produced by state-of-the-art 3D printing with submicrometer accuracy and a Fresnel zone plate fabricated by electron beam lithography and gold electroplating. These elements were produced in-house using the cleanrooms and nanofabrication facilities at Paul Scherrer Institut. The apochromatic X-ray optics was characterized by scanning transmission X-ray microscopy and ptychographic X-ray imaging. Sub-micrometer apochromatic focusing was achieved for an X-ray energy range from 7 to 12 keV. The X-ray measurements were carried out at beamline P06 at PETRA III, DESY (Hamburg, Germany).

In the future, apochromatic X-ray lenses could emerge a cost-effective and compact alternative to mirror-based systems, with the further advantage of being on-axis imaging optics. They are likely to play an increasingly important role in the field of X-ray imaging and microscopy and for their scientific applications in both accelerator-based and laboratory X-ray sources.