Soft X-Ray Polarimetry with Multilayer Optics
At BESSY there are 18 (out of 50) beamlines in operation which are dedicated for experiments with polarised soft x-ray radiation. Most of these beamline are coupled to an helical undulator source (6 in total) where the polarisation can be selected freely between linear (hor.-vert. or any angle) and circular (left - right-handed). Knowledge of the degree of polarisation at the experiment is of utmost importance for many users. We have 3 polarimeters with which we can measure the polarisation state of the light in an energy range between 10 and 1000 eV (F.Schäfers et al., Appl.Optics 38, 4074 (1999).
As polarising optical elements we use novel multilayers which have been developed in house and which are operated in transmission (as phase retarders, quarter-wave plates) and linear analysers. As a spin-off from a polarisation measurement not only the polarisation of the radiation is determined, but also the optical properties of the multilayers in use are characterised (self calibrating).
The polarimeter is also used for a varity of investigations including magneto-optics and multilayer characterizations.
Recent achievement is the development of an achromatic phase shifter on the basis of a W/B4C transmission multilayer for XUV broad-band polarization measurements at the magnetic 2p edges of Fe, Co and Ni. (A single W:B4C transmission multilayer for polarisation analysis of soft x-radiation up to 1 keV, M.A. MacDonald, F. Schaefers and A. Gaupp, Optics Express, submitted Oct. 2009)