A rotating-interface method for measuring optical constants of weakly absorbing medium at high-temperature

Abstract

Experimental identification method is a fundamental means of determining the spectral properties of semitransparent materials. A series of distinctly different and independent experimental data is usually required so as to provide multiple constraints to feed the retrieval model. However, for high-transparency and low-absorption materials, directional transmittance measurements may encounter unpredictable temperature gradients due to too many outlets on the heating compartment. And the transmittance measurements in a certain direction may confront the problem of low recognition that the transmittances of samples with different acceptable and reasonable thicknesses are undistinguished. To resolve these problems, a rotating-interface method coupling with an identifying procedure was proposed in this study. The rotating-interface method can provide a means of measuring various directional transmittances without additional outlets in the heating cell. There is barely any influence on the temperature field of the sample. Applied on multilayered specimens with various layers, this method could provide more independent and distinctly different transmittances by using as few specimens as possible. And thus the enough transmittance data was used as constraints to feed the retrieval model, guaranteeing the uniqueness and stability of the result. To verify this method, a C-cut sapphire was measured for the low absorption wavelengths throughout 0.35 to 1.1 μm at temperatures ranging from 300 to 1500 K. And then an experimental identification procedure was performed to retrieve the indexes of refraction and absorption. Considering the incident and detecting solid angles, a Monte-Carlo ray-tracing (MCRT) method was used to predict the experimental transmittances. A genetic algorithm was adopted to search and to optimize the association of refractive index and absorption index, which was brought in the retrieval model of least-square to minimize the difference between the transmittances, measured and predicted. Finally, temperature and wavelength dependence of transmission properties of the C-cut sapphire up to 1500 K were obtained.