Transmittance and optical constants of Tm films in the 2.75–1600 eV spectral range

The optical constants of thulium (Tm) films were obtained in the 2.75-1600 eV range from transmittance measurements performed at room temperature. Thin films of Tm were deposited by evaporation in ultrahigh vacuum conditions and their transmittance was measured in situ. Tm films were deposited onto grids coated with a thin, C support film. Transmittance measurements were used to obtain the optical extinction coefficient k of Tm films. The refractive index n of Tm was calculated using the Kramers-Krönig analysis. k data were extrapolated both on the high and the low energy sides by using experimental and calculated extinction coefficient values available in the literature. Tm, similar to other lanthanides, has a low-absorption band below the O 2,3 edge onset; the lowest absorption was measured at ∼23 eV. Therefore, Tm is a promising material for filters and multilayer coatings in the energy range below the O2,3 edge in which materials typically have an absorption stronger than away. Good consistency of the data was obtained through f and inertial sum rules.