Application of Waveguide Raman Spectroscopy to High-Index Dielectric Films

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Abstract
Waveguide Raman spectroscopy has been applied to the analysis of single-layer, refractory-oxide, thin-film coatings on fused silica. With the use of the film as a waveguide, the interaction of the laser probe beam with the film is maximized, and interference from the silica substrate is minimized. An amorphous film of Ta2O5 was found to be an excellent waveguide, yielding an intense Raman spectrum. Even though polycrystalline films of Y2O3, ZrO2, HfO2, and ThO2 were found to be poor waveguides, they still yielded Raman spectra containing useful structural information. Such Raman spectra showed that the ThO2 film was initially in an unusual structural form, which spontaneously transformed into cubic ThO2. Even for films yielding relatively weak Raman bands, substrate Raman scattering was not a serious interference. Representative spectra are presented, along with a brief discussion of the requirements for coupling optical beams into films with large refractive indices.