Original research articleDetermination of infrared refractive index of ZnS and YbF3 thin films by spectroscopy
Introduction
The thin-film materials are the basis for the preparation of optical films, and it is very important to accurately grasp the optical properties of thin film materials in the wide band range. The optical constants of the thin film are always affected by a number of factors such as deposition methods, process parameters and material properties [[1], [2], [3], [4], [5]]. At present, in order to obtain the optical constant of the thin film, firstly, the monolayer film is prepared under certain process conditions. Then, the optical constant of the films is solved by spectrometry (extreme value method, envelope method and full spectral fitting inversion method) [[6], [7], [8]], spectroscopic ellipsometry, prism-film coupler spectroscopy, surface Plasmon and polarization conversion, and others [[9], [10], [11], [12], [13]]. The full spectral fitting inversion method can obtain the optical constant of the thin film by fitting the transmittance or reflectance of the thin film. Furtherly, the inversion method requires only the transmittance or reflectance of the film, but not the relevant extreme point. Therefore, the inversion method is widely used [14,15].
In general, the YbF3 thin film is a typical porous film, and the packing density is not very high. The YbF3 thin film is exposed to air, and can adsorb water vapor, which may make its refractive index changed [16,17]. In the optical coatings, the YbF3 thin film is often wrapped in coatings, not directly exposed to the air. The refractive index obtained by the monolayer YbF3 thin film is different from that of the YbF3 thin film wrapped in the coatings. Therefore, the refractive index of the YbF3 thin film wrapped in the coatings has a practical significance for the design and preparation of optical coatings.
In this paper, using the Sellmeier dispersion model, the infrared refractive index of the ZnS thin film was obtained by fitting the 2.5 μm–11 μm transmittance of the monolayer ZnS thin film on the monocrystalline germanium substrate, and that of the YbF3 thin film wrapped in the coatings was obtained by fitting the 2.5 μm–11 μm transmittance of the YbF3/ZnS bilayer coatings on the CVD ZnSe substrate. The results can provide reference to design and develop the corresponding optical coatings elements.
Section snippets
Fitting refractive index of non-absorbing substrate
If the scattering of the substrate can be neglected, the double-sided transmittance of the non-absorbing substrate T0 can be expressed as:where R1 is the single-sided reflectivity of the substrate, and can be written as:where ns is the refractive index of the substrate, and the refractive index of air is 1. The refractive index of the substrate in the infrared band can be expressed by the Sellmeier dispersion formula, i.e.:
Experiment
The coating samples were prepared with the ARES1510 high vacuum coating plant produced by Leybold Optics Company and were deposited by thermal evaporation with molybdenum boats. The ZnS thin films were deposited on the monocrystalline germanium(Ge) substrates with the diameter of 30 mm and the thickness of 3 mm to calculate the infrared refractive index of the ZnS thin films. The YbF3/ZnS bilayer coatings were deposited on the CVD ZnSe substrates with the diameter of 30 mm and the thickness of
Refractive index of substrate
In this paper, the refractive indices of the CVD ZnSe and monocrystalline Ge substrates were calculated by using the φ30 mm × 3 mm samples. The transmittance of the samples were measured by the Spectrum GX FTIR spectrometer at room temperature (300 K). The refractive indices of the substrates were obtained by fitting the transmittance of the sample at the wavelength range of 2.5 μm–11 μm.
The Sellmeier dispersion coefficients of the CVD ZnSe substrate obtained by fitting transmittance were: A0
Conclusions
This paper presents a relatively simple and accurate method for determining the infrared refractive index of a thin film. If the absorption and inhomogeneity of thin films can be neglected, the method uses the Sellmeier dispersion model to determine the refractive index of the thin film by fitting the measured transmittance. The single layer ZnS thin film was prepared on the Ge substrate by thermal evaporation with molybdenum boat, and the refractive index of this film was obtained by fitting
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