Retrieving optical constants of semitransparent films from directional emittance at multiple angles

Abstract

The refractive index (n) and extinction coefficient (k), collectively known as optical constants, are critical parameters in the characterization and design of thin-film optical materials. However, precise determination of these constants for semitransparent thin films remains technically challenging, particularly due to the inherent limitations of conventional optical measurement techniques. In this study, we propose an emittance-based retrieval approach that incorporates wave interference effects within multilayer structures to accurately determine the optical constants. Titanium dioxide (TiO₂) thin films were selected as a model system. Directional emittance measurements were performed across the visible and near infrared spectrum (0.4 μm - 1.0 μm) at emission angles of 8°, 20°, and 30°. The corresponding optical constants were retrieved by solving a system of equations derived from the measured spectral data. The retrieved values demonstrated excellent agreement with those obtained from spectroscopic ellipsometry, yielding absolute errors of 0.024 for the refractive index and 0.013 for the extinction coefficient. These results validate the robustness and accuracy of the proposed method and highlight its potential as a reliable alternative for optical characterization of semitransparent thin films.