Multilayer reflective polarizers for the far ultraviolet

摘要

Polarimetry in the far ultraviolet (FUV) is a powerful tool for the interpretation of the role of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. FUV polarimetry from space provides more accurate observations on the kinetics of the features and on local magnetic fields through the Doppler and Hanle resonant electron scattering effects. Particularly interesting lines for FUV polarimetry are H Lyman a (121.6 nm) and P (102.6 nm), along with OVI lines at 103.2 and 103.8 nm. One key element to perform polarimetry measurements at these wavelengths is the need of efficient polarizers. A limitation of the available polarizers, such as crystal plates of MgF_2 and LiF working at Brewster angle, is their moderate reflectance at the non-extinguished component of the electric field, which results in a modest polarizer efficiency. Research is underway to develop efficient polarizers operating in the short FUV based on reflective multilayer coatings. First wavelength target is 121.6 nm. We have obtained promising preliminary results with (Al/MgF_2)_3 multilayer polarizers. Their reflectance for both s (TE) and p (TM) polarization has been measured as a function of the angle of incidence and wavelength at BEAR beamline of ELETTRA synchrotron. The analysis of the first campaign enabled refining the designs for a second campaign, which has resulted in efficient polarizers in a range around 121.6 nm. Future research will address the stability of these polarizers both in atmosphere and in relevant conditions for space optics.