Reflective and transmissive broadband coating polarizers in a spectral range centered at 121.6 nm

Polarimetry 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. One of the key lines for observations is H I Lyman α (121.6 nm) among few spectral lines in the far ultraviolet (FUV), and hence efficient linear polarizers at this line are demanded. New designs based on (Al/MgF2)n multilayer coatings have been developed to obtain the smallest possible reflectance in the parallel plane of polarization (Rpar) with a simultaneous high reflectance in the perpendicular plane of polarization (Rper). Samples stored in nitrogen for ~8–17 months resulted in efficient polarizers at 121.6 nm, with Rpar ~ 0.01–0.017 and Rper ~ 0.69–0.725. The designs with a number n = 3–4 bilayers of Al/MgF2 result in a wider spectral range of efficient linear polarizers, compared to what can be obtained with n = 2. Coatings following various designs with good polarizing performance in a 7–8 nm wide FUV range were prepared. For the first time, a transmissive coating polarizer has been developed for this range, which has the benefit that it involves no deviation of the beam; it is based on another design of (Al/MgF2)3 multilayer coating. The transmissive polarizer has a good transmittance ratio between the two polarization components and, even though its figure of merit is not as high as that of the reflective polarizers, it incorporates filtering properties to reject wavelengths both below and above 121.6 nm; this property might enable a polarimeter for solar physics with an improved global figure of merit if a filter to isolate the H I Lyman α line could be avoided.