Multilayer-based Soft X-ray Polarimetry

An overview about the soft x-ray polarimetry work done at BESSY over the last 10 years is given. At BESSY ten elliptical undulator beamlines are operating in the VUV and soft x-ray range which enable the polarisation state of the synchrotron radiation to be changed from linear (horizontal or vertical) to left- or right-handed circular. It is essential that the degree of polarisation is quantitatively known, since this is a normalization quantity for many polarisation-sensitive experiments (e.g. MCD-spectroscopy). For a polarimetry experiment i.e. the measurement of the complete polarisation state of light two optical elements are required acting as phase retarder and as linear polariser, resp. In the soft x-ray range specially tailored multilayers (ML) operating in transmission and in reflection have been developed and optimized for this purpose. By matching the ML-parameters (period, thickness ratio) to an absorption edge of one of the constituent materials a resonantly enhanced polarisation sensitivity can be achieved. Thus, ML-polarimetry is strongly connected with at-wavelength metrology of these polarisation optical elements for which the instrumentation and results are presented. Examples on magneto-optical spectroscopy and-polarimetry to determine properties of magnetic thin films or optically active substances are also presented (Faraday-, Kerr-effect, L-MOKE). Another topic which will be covered briefly is the HIgh Kinetic Energy photoelectron spectroscopy (HIKE) employing hard x-rays which recently came up and which is ideally suited to study – sample preserving- deeply buried layers or bulk properties of matter. An example for its application to characterise ML interfaces is shown