The activities in X-ray optics focuses on the fabrication and characterization of compound refractive X-ray lenses as well as on grid structures for X-ray interferometers.
X-ray lenses are needed, for example, to build X-ray microscopes whose resolution may be far above that of light-optical microscopes. In X-ray spectroscopy, X-ray lenses are used for focusing an X-ray beam to an as small as possible spot on the surface of the sample to be analyzed.
The refractive power of materials in relation to that of air or vacuum is the higher the more the refraction index deviates from 1. Since the difference is very small, i.e. approximately Δn=10-5 – 10-7, in the case of X-rays, considerable focusing can only be achieved by stacking a large number of strongly curved lenses behind one another. Using deep X-ray lithography, rows of such lens structures are fabricated in X-ray-stable polymers (for energies in the range of 5 – 50 keV) or in nickel (50 – 500 keV) on substrates in one step without time-consuming individual-lens adjustment. While 1D lenses for line focus generation are fabricated within one irradiation step, lens systems that focus X-ray light in one point are obtained by crossing cylindrical lenses by 90° due to two inclined irradiation steps.
Focal spot diameters below 100 nm have been obtained so far by means of IMT X-ray lenses. The apertures of parabolic lenses are in the range of up to 300 µm. The focal distances are only a few centimeters.
Current research activities are dedicated to
- reducing focal spot diameters below 50 nm by use of adiabatic lenses,
- enlarging the aperture to above 1 mm by means of prismatic or mosaic lenses and rolled-type lenses,
- demonstrate the use of the X-ray lenses in X-ray tubes,
- developing X-ray microscopes with optimized condenser and imaging lenses for synchrotron sources and X-ray tubes.
Schematic view of a “nanoscope” (or X-ray microscope).
Today, X-ray images are produced using the absorption contrast method which, depending on the object analyzed, requires relatively high irradiation doses and is only little sensitive to smaller variations in the density of specimens. Alternative approaches applying phase contrast methods based on Talbot interferometry make use of phase shifts genarated by the sample. The setups needed require phase gratings for defined phase variations, and amplitude grids with extreme aspect ratios (> 100) to absorb parts of the high-energy radiation. Non-coherent sources, moreover, require additional so called source or coherence grids.
The current research is devoted to
- developing processes for manufacturing such grids with periods in the range of a few micrometers and with aspect ratios > 100,
- analyzing effects of the quality of grids on the quality of X-ray images,
- adapting grid geometries to ray trajectories (grid curving),
- enhancing grids to make, e.g. tomography, mammography, or materials testing methods accessible to Talbot interferometry.
Talbot interferometric phase contrast imaging by means of