Waveguide refractometry as a probe of thin film optical uniformity
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Waveguide refractometry as a probe of thin film optical uniformity B. G. Potter, Jr., D. Dimos, and M. B. Sinclair Sandia National Laboratories, Albuquerque, New Mexico 87185-1349 (Received 6 March 1996; accepted 12 August 1996)
Optical inhomogeneities through the thickness of a sol-gel-derived, spin-coated Pb(Zr,Ti)O3 (PZT) thin film have been evaluated using prism-coupled waveguide refractometry. Unusual waveguide coupling angle behavior has been treated using a multilayer model to describe the optical characteristics of the film. Waveguide refractometry measurements, performed after incremental reductions in film thickness, were used to develop a consistent model for optical inhomogeneity through the film thickness. Specifically, a thin film layer model, consisting of alternating layers of high and low refractive index material, was found to accurately predict irregularities in transverse-electric (TE) mode coupling angles exhibited by the film. This layer structure has a spatial periodicity that is consistent with the positions of the upper film surface at intermediate firings during film synthesis. The correlation emphasizes the impact of the multistep thin-film deposition approach on the optical characteristics of the resulting thin film.
I. INTRODUCTION
Lead zirconate-titanate (PZT) materials exhibit a wide variety of electro-optical behaviors stemming both from electric-field induced realignment of ferroelectric domains and the intrinsic electro-optical response of the crystal itself.1 In particular, PZT thin films offer the promise of small, robust, integrable elements for applications such as waveguide-based optical modulators and switches, spatial light modulators, and read/write nonvolatile optical memories.2–4 Successful utilization of these materials for optical applications is dependent not only on the electro-optic response of the films, but also on their passive optical properties, e.g., degree of optical scattering and guided-wave optical field profiles. Typically, film thicknesses . 1 mm are desired to increase the optical pathlength or sustain waveguiding modes. Consequently, a multilayer sol-gel deposition approach is necessary to synthesize ferroelectric thin films of sufficient thickness for these applications. It is important, therefore, to determine if this process leads to optical inhomogeneities in the film that could adversely affect optical performance. An accurate identification and characterization of any inhomogeneity is, therefore, required. Waveguide refractometry is a powerful technique for the determination of thin film optical constants such as refractive index (n) and thickness (d) in both single and multilayer structures. The approach has been described elsewhere5,6 and consists of the measurement of reflectivity with incidence angle from a thin film sample which is placed in optical contact with the base of a high refractive index prism. The prism allows 546
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J. Mater. Res., Vol. 12, No. 2, Feb 199
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