Multilayers and optical constants of various fluorides in the far UV

Luis Rodríguez-de Marcos; Juan I. Larruquert; José A. Méndez; José A. Aznárez

Abstract

Multilayer coatings in the far UV (FUV) are required for various fields of application, such as space instrumentation for astrophysics, solar physics and atmosphere physics, as well as free electron lasers, plasma diagnostics, synchrotron radiation, spectroscopy, etc. In order to design the desired multilayer, accurate optical constants of the materials are required. Several fluorides are among the materials in nature that keep their transparency down to the short FUV, which makes them almost the only choice for multilayer coatings tuned in the ~100-150-nm wavelength range.

The optical constants of thin films of MgF₂, LaF₃, and CeF₃ have been determined in the spectral range of 30-950 nm. Among them, MgF₂ is a low refractive index material whereas LaF₃ and CeF₃ have a relatively high refractive index at short wavelengths; this contrast is adequate to make multilayer coatings. Fluoride thin films were deposited by evaporation onto substrates at 523 K. Optical constants were calculated using sets of transmittance, reflectance, and ellipsometry measurements. The measured optical constants were extended to a broader range with literature data and extrapolations in order to obtain self-consistent sets of data using the Kramers-Krönig analysis. The optical constant data here presented extend the available literature data both shortwards and longwards, particularly for CeF₃ where few data had been reported.

The obtained optical constants of MgF₂ and LaF₃ were used to design narrowband reflective multilayer coatings for the short FUV. Multilayer coatings centered at 121 and at 130 nm with remarkable reflectance were prepared. The coatings kept a valuable reflectance after ageing in a desiccator for 12 months.