Narrowband filters for the FUV range

Space observations in the far ultraviolet (FUV, 100-200 nm) are aimed at providing essential information for astrophysics, solar physics, and atmosphere physics. There are key spectral lines and bands in the FUV for the above disciplines. Despite various developments in the recent decades, yet many observations are not possible due to technical limitations, of which one of the most important is the lack of efficient optical coatings. Hence for solar physics applications there are needs of narrowband coatings for key wavelengths such as H Lyman β (102.6 nm) and OVI lines (103.2 and 103.8 nm). For atmosphere physics, narrowband coatings are required for observations at spectral lines such as OI (135.6 nm) and at the N2 Lyman-Birge-Hopfield band (LBH, 127-240 nm). In solar corona observations, often the intensities of the target lines are weak, and this radiation may be masked by more intense lines, such as H Lyman α at 121.6 nm. Until now, no narrowband multilayers peaked in the ~100-105 nm range have been reported, which is due to the absorption of materials at these wavelengths. When efficient narrowband coatings are not possible, an option is the use of coatings with high reflectance at the target wavelength and simultaneously low reflectance at the undesired wavelength, such as Lyman α. We have developed novel multilayers to address this target, with combinations of these materials: Al, LiF, SiC and C. We developed multilayers based on the following three systems, Al/LiF/SiC, Al/LiF/SiC/C, and Al/LiF/SiC/LiF. Their reflectance was measured both when fresh and after some storage in a desiccator. Al/LiF/SiC and Al/LiF/SiC/C systems displayed a high Lyman β/Lyman α reflectance ratio when fresh, although they resulted in an undesired reflectance increase at Lyman α for the aged samples and the reflectance ratio Lyman β/Lyman α became small; this behavior turned these systems useless for the present application. The most promising multilayers were the ones based on the Al/LiF/SiC/LiF system, which resulted in a good performance and a limited evolution after months of storage in a desiccator. Five samples based on the Al/LiF/SiC/LiF system were prepared and measured in the 50-190 nm spectral range. These samples resulted in high reflective and narrowband coatings peaked at 100-101 nm, with a promising reflectance ratio Lyman β/Lyman α when fresh. Some efficiency degradation was observed after the sample storage in a desiccators; however all samples retained a narrowband performance over time and a high Lyman β/Lyman α ratio. The same system can be designed to be an efficient narrowband coating peaked in the target spectral range and not constrained to a specific performance at Lyman α. Hence an 8-month aged sample exhibited a reflectance as high as 61% at the peak wavelength of 100.9 nm, at near-normal incidence, the highest experimental reflectance reported in this range for a narrowband coating. We have also prepared narrowband transmission coatings tuned either at 135.6 nm or at the center of the LBH band (~160 nm), with the requirement to strongly reject the out-of-band, particularly the visible and close ranges. The coatings were based on (Al/MgF2)n multilayers, with n ranging between 3 and 4. The coatings were successful at rejecting the visible, with a transmittance lower than 10-5. The transmittance at the peak was 0.087 for coatings stored in a desiccators for 13 days.