Optical Properties of Various Evaporated Rare Earth Oxides and Fluorides*

Abstract

To furnish a greater choice of film materials for application in optical multilayer coatings, the optical properties of evaporated rare earth compounds such as Y₂O₃, La₂O₃, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Gd₂O₃, LaF₃, NdF₃, and CF₃ were studied in the wavelength region from 0.22 micron to 2 microns. The materials were evaporated from tungsten boats and condensed on glass and fused quartz substrates at various temperatures. The optical constants n and k were determined mainly from reflectance, transmittance, and true thickness measurements. The following oxides were found to be useful because of their low absorption in the visible: La₂O₃ with n ranging from 1.85 to 1.95 with increasing thickness, Pr₆O₁₁ with n ranging from 1.92 to 2.05, also with increasing thickness, and Nd₂O₃ with n=2.05 for films of sufficiently low absorption. LaF₃ (n=1.60) and NdF₃ (n=1.61) showed no appreciable absorption in the whole wavelength region studied; CeF₃ (n=1.63), however, exhibited two weak absorption bands at 0.234 and 0.248 microns. All n values are given at a wavelength of 0.5 micron and for unbaked films deposited at 300°C substrate temperature. The oxides condensed as almost amorphous films, whereas the fluorides developed fairly large grains as indicated by their sharp electron diffraction rings. All films were hard and showed excellent chemical and mechanical durability.