Efficient Imaging and sub-25 nm Focusing using crossed Multilayer Laue Lenses with hard X-ray Energies

Multilayer Laue lenses (MLL) are an emerging type of hard X-ray optics. Calculations have shown their potential to reach focal spot sizes in the order of the wavelength of the incident radiation. In order to achieve these resolutions, it is necessary to manufacture optics with sufficient numerical apertures.
For focusing applications it is desired, that the working distance of such a lens is sufficiently large in order to contain the samples to be measured and ideally providing working space to allow for tomographic measurements or dedicated sample environments. This requires working distances in the order of several millimeters and more.
At Fraunhofer IWS several designs of MLL have been projected and manufactured. The two recent types feature focal lengths of 9 mm and 45 mm at an X-ray energy of 12 keV with working distances for focusing applications of 3.1 mm and 28 mm and theoretical diffraction limited focal spot sizes of 18 nm and 40 nm, respectively. One single MLL is a 1D focusing/imaging optical element and two MLLs have to be used in a crossed geometry in order to achieve point focusing/imaging. Two MLLs can be prepared to allow for switching between point and line focus in a matter of seconds by the illumination of one lens only. Using a line focus allows enhancing the effective flux on suitable 2D-samples, such as cross sections of coatings grown on flat substrates.
Focusing experiments have been performed at the ESRF beamline ID13. Best results have been achieved with sub 25 nm FWHM focal profiles in horizontal and vertical directions for a crossed MLL setup. The best efficiency achieved was more than 20% for this pair and 44% for a single lens and maximum flux on sample was measured to be 2.7×1010 photons/s at a photon energy of 13 keV. Full field diffraction imaging experiments have been demonstrated at ESRF ID01. At an X-ray energy of 19.7 keV in situ experiments have been performed with a sample to lens distance of up to 74 mm.
Efficiency calculations have been done for energies up to 100 keV X-ray energy and show, that the lens can be used efficiently even at significantly higher energies. As focal length and working distances scale linearly with the energy even lenses with short focal lengths at X-ray energies around 12 keV are good to use for experiments at X-ray energies above 50 keV.
Current challenges in the lens development remain to enhance the effective aperture of the lenses. Currently depositions up to 40-70 µm are deposited at Fraunhofer IWS Dresden regularly. A model multilayer system with a thickness of 100 µm has been obtained by exploiting a dedicated three material multilayer system made of Molybdenum, Carbon and Silicon. Further refinement is necessary in order to keep the resulting phase errors acceptable over this large growth thickness.
The aim of the development is a single optical element housing two crossed lenses, which can be used within existing beamline infrastructures with no or little modifications within the setup.