VUV mirrors for the (80-120 nm) spectral range

The observation of hot plasmas in the interstellar medium requests efficient mirrors in the 80-120 nm wavelength range. Contrary to that of most metals, the high reflectivity of pure aluminum is maintained close to 80% in this range. Unfortunately, it is drastically reduced to values lower than 10% by the strongly absorbing thin alumina layer which spontaneously forms itself upon air contact. Usually the deposition of thin fluorides films on aluminum (MgF₂ or LiF) is used to prevent the alumina formation and consequently extend the high reflectivity range from visible towards shorter wavelengths. But this approach works only from the alumina to the fluoride bandgap (λ ≈ 100 nm). Other materials were studied by Larruquert who reported a measured reflectivity as high as 34% at l = 90 nm by using a Al/MgF₂/SiC thin films stack. The aim of the investigation reported here is to define and test different and original aluminum based thin films stacks which optimize the mirror reflectivity in the 80-120 nm range. We present the results of our simulations from a large number of materials and the first experimental tests of the predicted best solutions.