Instituto de Óptica «Daza de Valdés»
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Luis Rodríguez-de Marcos, Juan I. Larruquert, José A. Aznárez, Manuela Vidal-Dasilva, Sergio García-Cortés, José A. Méndez, Luca Poletto, Fabio Frassetto, A. Marco Malvezzi, Daniele Bajoni, Angelo Giglia, Nicola Mahne, and Stefano Nannarone

Abstract

Strontium (Sr) is a material with low-absorption bands in the extreme ultraviolet (EUV), which makes it a potential candidate for band pass filters and multilayer coatings. Yet, a better knowledge of the optical properties of Sr is required for these developments. The optical constants n and k of Sr thin films have been obtained in the 6–1220 eV range from transmittance measurements performed at room temperature. These are the first experimental optical constant data of Sr in most of the range. Thin films of Sr with various thicknesses were deposited by evaporation in ultrahigh vacuum conditions, and their transmittance was measured in situ. Sr films were deposited onto grids coated with a thin C support film. Transmittance measurements were used to directly obtain the extinction coefficient k of Sr films. The refractive index n of Sr was calculated with Kramers-Krönig analysis. For this, k data were extrapolated both on the high- and on the low-energy sides by using experimental and calculated k data available in the literature. It was found that, similar to other alkaline-earth metals, Sr has a low absorption band in the EUV, which for Sr is below its N2,3 edge, with a minimum at ∼18.5 eV, a range where most materials in nature have a large absorption. A second spectral range of interest for the low absorption of Sr is below its M4,5 edge at 132 eV. In spite of these remarkable properties, Sr is a very reactive material and the stability of coatings encompassing Sr may be an issue. Good consistency of the data resulted from the application of f and inertial sum rules.