Spectral and polarization properties of VUV-mirrors for experiments at a HHG beamline

Paper in proceeding, 2020

For polarization-resolved reflection experiments such as ellipsometry, not only is high reflectivity in a wide spectral range required for mirrors but also the quality of their polarization response is important. Furthermore, for VUV ellipsometry, optimal angles of incidence at the sample are between 45° and 60° with respect to the surface normal. In the best case, a setup should even allow variable angles. This requires reflective optics working at non-grazing incidence. In this theoretical study, a selection of potentially relevant materials and mirror designs for broadband use in the VUV is investigated. Based on the available tabulated databases of optical constants, we performed transfer-matrix calculations to obtain reflectance as well as polarization-response spectra in the desired VUV range up to 50 eV. From the variety of materials, we discuss metals that are otherwise commonly used at grazing incidence, Si, SiC, and as representatives for coating layers MgF2, SiO2, and Al2O3. While SiC is most universal, Si with only native oxide layer performs well especially below 25 eV. Aluminum has good properties but oxide layers are very detrimental, as protective coatings are in general.