Optical waveguide measurements of reactive low voltage ion plated thin films

Abstract

Thin films are commonly used to make optical coatings. When used for this purpose it is important that the films have low absorption and scattering losses. Reactive low voltage ion plating (RL VIP) is a relatively new method for making optical thin films. This process produces stable, high-index coatings with low losses even though the process is not fully understood yet / Gue90b/. I will discuss the measurements made on RL VIP films of four materials: Ti 02, Ta2O5 , A12O3 , and Si 02• The measurements are part of an effort to improve the quality of the coatings made by the RLVIP process. I used a waveguide approach to measure the index of refraction, thickness, and loss of the films /Tien70, Ulr73/. With this method the guided optical beams can travel several centimeters in a film which allows for the determination of the loss and scattering phenomena over a large area. Because of the greater interaction distance the waveguide method is capable of measuring lower losses with greater sensitivity than is possible with transmission measurements. Optical techniques, such as ellipsometry and transmission spectroscopy, can also be used to determine the film properties; however, some of these techniques do not provide accurate values of a single parameter or involve making allowances for substrate effects. Thus, for some films the standard optical techniques may have limited accuracy /Wes79/. Of the several waveguide coupling techniques possible we chose to use a prism-coupler because it is non-destructive and does not require any special film preparation.

The waveguide attenuation (loss) is determined from the light scattered out of the plane of the film. The scattered light is detected by a charged-coupled device (CCD) camera. The images of the scattering waveguide streaks are stored digitally and analyzed with the aid of a computer. We have measured losses that ranged from < 1 dB/ cm to - 100 dB/ cm with a repeatability of better than 5 % . The effects of optical misalignment was investigated to determine the accuracy and repeatability of the attenuation measurements. Defocusing the camera by changing the distance between the sample and camera and tilting the camera so that its optical axis is not perpendicular to the sample normal were found to have a large effect on the result of the attenuation measurement. In addition to the total attenuation, one might also want to know how much light is being scattered from the bulk of the film and how much from its surfaces. A number of theories have been published that relate the effects of surface roughness and refractive index fluctuations in the volume of the film to waveguide attenuation. A common method using boundary-perturbation theory has been described by Ames and Hall /Ames83/ and by Walter and Houghton /Wal78/. Imai et al. /Imai81/, using a perturbation technique, found that the far-field radiation patterns could be formed by an appropriate combination of surface and volume effects. I describe a method that allows for the calculation of the fraction of the total loss caused by the surfaces or the bulk of the film. The method is based on an electromagnetic field analysis described by Channin, Hammer, and Duffy /Cha75/ in 1975. It requires the precise measurement of the coupling angle, q.v., and the total attenuation of two or more waveguide modes. The coupling angles are used to calculate he electric field profiles of the modes. I have applied the theory to the data of thermally evaporated ZnS thin films, as well as TiOi and Ta2O5 deposited by RL VIP, and report the results here.

Notes

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Graduation Date

1991

Semester

Fall

Advisor

Guenther, Karl H.

Degree

Master of Science (M.S.)

College

College of Arts and Sciences

Department

Physics

Format

PDF

Pages

58 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0029057

Subjects

Arts and Sciences -- Dissertations, Academic; Dissertations, Academic -- Arts and Sciences

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