Polarizers tuned at key far-UV spectral lines for space instrumentation

Abstract

Polarimetry is a valuable technique to help us understand the role played by the magnetic field of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. Polarimetry in the far ultraviolet (FUV: 100-200 nm), which must be performed from space due to absorption in terrestrial atmosphere, supplies fundamental data of processes that are governed by the Doppler and Hanle effects on resonantly scattered line-emission. To observe these processes there are various key spectral lines in the FUV, from which H I Lyman α (121.6 nm) is the strongest one. Hence some solar physics missions that have been proposed or are under development plan to perform polarimetry at 121.6 nm, like the suborbital missions CLASP I (2015) and CLASP II (2018), and the proposed solar missions SolmeX and COMPASS and stellar mission Arago. Therefore, the development of efficient FUV linear polarizers may benefit these and other possible future missions. C IV (155 nm) and Mg II (280 nm) are other spectral lines relevant for studies of solar and stellar magnetized atmospheres.High performance polarizers can be obtained with optimized coatings. Interference coatings can tune polarizers at the spectral line(s) of interest for solar and stellar physics. Polarizing beamsplitters consist in polarizers that separate one polarization component by reflection and the other by transmission, which enables observing the two polarization components simultaneously with a single polarizer. They involve the benefit of a higher efficiency in collection of polarization data due to the use of a single polarizer for the two polarization components and they may also facilitate a simplified design for a space polarimeter. We present results on polarizing beamsplitters tuned either at 121.6 nm or at the pair of 155 and 280 nm spectral lines.

Linear polarizer design for application in the far-ultraviolet spectral range

“…This paired with its ability to deliver small contrast between Rs and Rp polarizations, thereby minimizing the impact on the throughput of the 4-mirror polarizer. Figure 3 ) [10] and the throughput (𝑇 𝑠 ) [9]. All calculations were performed with all mirrors at an angle of incidence of 57°.…”

“…Alternative designs with polarization capabilities at wavelengths <120 nm have been reported, such a 3-mirror polarizers [4,5], a pile of LiF plates [6,7], and coating polarizers [8,9,10] Figure 1. Side profile showing the configuration of the 4-mirror linear polarizer's design.…”

Nanowire Grid Reflecting Polarizers for Ultraviolet Applications

“…For instance, a fair transmissive linear polarizer tuned around 121.6 nm was obtained by realizing a stack of 3-4 bilayers of Al/MgF 2 [21]. Moreover, by exploiting the high reflectance of aluminum in this spectral range, a polarizing beam-splitter able to simultaneously select one polarization component by reflection and the perpendicular component by transmission has been fabricated [22]. The idea of adopting polarizing films working in reflection may give additional benefits.…”