Antireflection coatings combining interference multilayers and a nanoporous MgF2 top layer prepared by glancing angle deposition

Thin MgF2 films are prepared using the glancing angle deposition technique in order to produce optical thin film materials with low refractive index. The optical and structural film properties are examined by spectroscopic ellipsometry and scanning electron microscope. Results show that the films are transparent with a physical thickness of 162 nm. The films present a porous and inhomogeneous microstructure in which the porosity and thus the refractive index vary inside the film. The film effective refractive index is found to decrease from 1.33 near the substrate to 1.16 at the film surface. The dependence of the refractive index on the film depth is determined using an optical model based on the effective medium approximation. In order to take into account the index gradient character of the films in the coating design of broadband antireflection coatings, the index function is discretized in a finite number of homogeneous layers. The nanoporous MgF2 layer is combined as an outermost layer with a multilayer system prepared by magnetron sputtering to manufacture a super-broadband antireflecton coating designed at a normal light incidence angle for the 400-1700 nm spectral range. Results show that the nanoporous MgF2 coated multilayer system prepared on BK7 glass exhibits excellent antireflection properties with a residual average reflection of 0.5% in the spectral range of 400-1700 nm, owing to the low refractive index of the MgF2 upper layer. (C) 2014 Elsevier B.V. All rights reserved.