X-ray Interferometry
The beam path in an LLL X-ray interferometer runs in the same way as in the Mach-Zehnder interferometer being used in light optics. Three thin crystal lamellas (LLL) constitute the X-ray-optical components at which the X-rays are - by means of Bragg reflection - in the first lamella split up, in the second deflected and in the third superposed for interference. As the magnitudes of the X-ray wavelength and the lattice parameter are of comparable order, the splitting angles are relatively large and the interference pattern reacts sensitively to relative movements of the crystal lamellas. This characteristic is made use of for the measurement of the lattice parameter in the unit of length "metre". At present, the achieved measurement uncertainty amounts to 12 attometers (10-18 m).
The interferometers calibrated in this way are, in length and angle measurements, used as vernier scale, for example of optical interferometers in the sub-nanometer and sub-nanorad measurement range. The first calibration station of this kind was established as COXI (combined optical and x-ray interferometry for high precision dimensional metrology) at the National Physical Laboratory in Great Britain, in cooperation with PTB and IMGC and with the financial aid of the EC. The advancing finishing accuracy in production engineering compels metrology to achieve ever smaller measurement uncertainties - a task also for X-ray interferometry, i.e. to develop an atomic length scale. The construction of these silicon interferometers is supported by finite element simulations of their mechanical properties. The crystals are processed in cooperation with the section "Ultra-precision Processing" of PTB's "Scientific Instrumentation" Department.
The interferometer shown in the figure is currently used to investigate measurement tasks in nanotechnology as, for example, transfer length standards for scanning tunnel microscopy and interpolation procedures in light interferometry.
The determination of fundamental constants as, for example, the fine-structure constant, and of the physical properties of solids as, for example, absorption edges, emission spectra, QED tests etc., requires the exact knowledge of X-ray wavelengths. Up to now, the wavelengths have been traced back to the numerical value of a calibrated silicon lattice parameter by measurements of Bragg angles. The relative measurement uncertainty achievable with this method is, however, limited by that of the lattice parameter to 1 . 10-8 at the best. It is the objective of a research task to develop a Fabry-Perot interferometer for visible and X-ray radiation which allows suitable reference wavelengths in the X-ray range to be linked up with the wavelengths of optical frequency standards with a relative uncertainty of <10-10 .