The observation of hot plasmas in the interstellar medium requires 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 upon air contact. We report here the results obtained with a capper layer of ZnSe. The optical indices given for this material by Palik's tables lead to predict a resulting high reflectivity, provided the layer prevents oxidization of underlying Al. The measured reflectivity does not agree with theory. The reasons for this inconsistency are examined. It is shown that complex indices of ZnSe in the wavelength region between 80 and 140 nm can be extracted. from the reflectivity measurements obtained with different ZnSe thicknesses on Al. The imaginary part of the index is then found to differ strongly from Palik's tables value.
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.
Excellent electro-optic properties of SBN crystals explain the attempts to control the deposition of high ordered SBN thin films with the aim of optical waveguiding and processing integration. We have reported in this paper the first epitaxial growth of SBN on MgO substrates using a sputtering method. The SBN stoichiometry approach and single phase preparation have been shown to be possible on the basis of the deposition mechanisms analysis. In spite of the XRD unreliability as an experimental guide in the complex case of SBN growth, the occurrence of a strong orientation of the SN parasite phase on MgO is shown to provide an efficient XRD test of the stoichiometry approach. Epitaxial SBN thin films are obtained which exhibit two in-plane orientations mirror symmetric to the MgO cell axis.