FIG. 1. Refractive index (λ=0.S5 μ) of CeF3-ZnS films as a function of the mixing ratio.

Refractive Indices of Evaporated Cerium Fluoride-Zinc Sulfide Films*

SHIRO FUJIWARΛ

Nippon Kogaku K.K. {Japan Optical Industry Co., Ltd.), Shinagawa-ku, Tokyo, Japan

(Received 10 June 1963)

IN a previous paper,1 it was shown that thin films with con­tinuously varying refractive indices ranging from 1.60 to 2.13

could be obtained by evaporating a mixture of cerium dioxide and cerium fluoride in various mixing ratios. In the course of later experiments it was found that the mixture of cerium fluoride and zinc sulfide could also be evaporated successfully for the same purpose. The experimental techniques of depositing the films and

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measuring the refractive indices were similar to those described in the previous paper except that the evaporation was carried out by using a radiation-heated vapor source.- From a practical point of view, the CeF3-ZnS mixture had an advantage over the CeF3-CeO2 mixture in deposition practices, as the evaporation temperature of the former (~1500°C) is lower than that of the latter (~1900°C). However, the CeF3-ZnS film was a little less durable than the CeF3-CeO2 film.

The measured refractive indices of CeF3-ZnS films at 0.55 μ as a function of the mixing ratio is shown in Fig. 1. It is seen from this

FIG. 2. Reflectance curves as a function of wavelength: (a) calculated and (b) measured reflectances for a quarter-half-quarter triple-layer coating (n1=1.38, n2 =2.20, n3=1.70, n4,=l.52, match point wavelength: 500 mμ); (c) measured reflectance for a single-quarter coating (n1=1.38, ng=1.52); (d) calculated reflectance for uncoated glass (ng=1.52).

figure that the indices are increasing continuously from 1.58 to 2.40 and that the reproducibility of the indices in three different depositions for each mixing ratio is sufficient for practical purposes. The durability of the film appeared to increase with the increment of CeF3 contents. In fact, though the ZnS film was of very low resistance to humidity, the CeF3-ZnS (CeF3>20% in weight) films did not deteriorate after the resistance test in which the film was boiled in distilled water for 15 min.

As an example of a practical application, a triple-layer anti-reflection coating (quarter-half-quarter wavelength3) was de­posited onto a glass plate (ng=1.52). The filming materials used were as follows: the layer next to air was MgF2(n1 = 1.38), the middle layer was CeO2(n2 = 2.20), and the layer next to glass was a CeF3-ZnS mixture ( N 3 = 1 . 7 0 ) . In Fig. 2, Curves (a) and (b) are the calculated and measured reflectances of this antireflection coating, respectively, Curve (c) is the measured reflectance of the single-quarter coating with n1 = 1.38, and Curve (d) is the calcu­lated reflectance of uncoated glass with ng=1.52. The disagree­ment between (a) and (b) in the shorter wavelength region may l)e attributed to the neglect of the dispersion of films in the calculation.

A study is now being made of the structure of evaporated CeF3-ZnS films by using the electron diffraction method.

* Patent pending. 1 S. Fujiwara, J. Opt. Sec. Am. 53, 880 (1963). 2 L. Holland, Vacuum Deposition of Thin Films (Chapman and Hall,

Ltd., London, 1956), Chap. 4, p. 134. 3 J. T. Cox, G. Hass, and A. Thelen, J. Opt. Soc. Am. 52, 965 (1962).