A Roadmap Towards Nanometre Size Hard X-Ray Focusing with Reflective Optics
Oliver Hignette
European Synchrotron Radiation Facility
Graded multilayers seem the most promising approach to achieve high numerical aperture and therefore small focused spot size with reflective optics. The theoretical limit with a classical Kirkpatrick Baez architecture is predicted at about 4 nanometre FWHM independently of the energy. This is assuming that the volume diffraction is behaving as a perfect stigmatic surface, and that the complex electromagnetic field interaction does not limit the final resolution. A proper theoretical description of all the phenomena involved is still lacking, but a multilayer focusing device has already been tested at ESRF with a 40-nanometre focus size. Despite a far from perfect mirror figure, the error budget analysis does not show experimentally any broadening due to volume interaction. Devices and experiments are planned at ESRF to further validate experimentally this behaviour.
A review of the technologies and methods, which have to be developed in the short term to get a predictable supply of the needed devices in the coming years, is presented. This includes both fabrication and metrology processes, with a level of sophistication, which most likely justifies pooling resources at a continental or worldwide level. This platform could be the base allowing reaching the theoretical limits by continuous increments. Once the single reflection architecture is mastered, multiple reflection schemes can be used to further increase the numerical aperture and reduce accordingly the spot size.
