UC San Diego

Polycrystalline materials have shown promise in advanced optical applications becauseof their unique optical and mechanical properties. As transparent polycrystalline materials become more important in optical applications, evaluation of their transmission across a wide range of wavelengths is crucial to device design. Most polycrystalline materials have non-uniform optical properties due to residual porosity, secondary phases, and/or crystalline anisotropies (e.g. birefringence). To better understand and describe transmission in these materials, an in-line transmission model is developed that incorporates reflection, scattering, and absorption losses. The model was fit onto transmission data from three polycrystalline material systems: ruby, yttria stabilized zirconia, and terbia. Parameters extracted from these fits can be used to describe wavelength dependent transmission with one simple analytical expression. The fit can also be used to decouple absorption form scattering, allowing for the extraction of important properties such as absorption coefficients. Another unique challenge to modeling transmission by transparent ceramics is modeling the scattering losses from birefringence. Even in the case of a single-phase polycrystal with negligible porosity, birefringence scattering will always be present whenever the crystal is anisotropic (non-cubic). Multiple models for birefringence scattering have been suggested in the literature that are generally successful in describing scattering loss in specific material systems. However, these models do not readily extrapolate to new material systems and their connection to birefringence scattering are never fully justified. We remedy this by deriving from Maxwell’s microscopic equations a first-principles model of birefringent scattering that is applicable to any single-phase, unaligned transparent polycrystal. This is achieved by first developing a framework for describing light propagation in dielectric solids by perturbing Maxwell’s equations These expressions are then used to develop a birefringence scattering model in ceramics. The derivation culminates in an equation which describes the birefringence scattering coefficient spectra using the single-crystal refractive index tensor and a representative polished surface micrograph. This model should be useful for designing materials and characterization. For example, by generating hypothetical transmission for a polycrystal with varying grain size or by characterizing average grain size using a transmission measurement.