Optical constants retrieval from a thin film at elevated temperatures using emittance

The refractive index and the extinction coefficient (optical constants) are essential in photonic design and thermal radiation utilization. These constants vary with the material phase, temperature, wavelength, and subject dimension. However, precisely retrieving these constants from a thin film is challenging at elevated temperatures and therefore temperature dependency is usually neglected. To tackle this challenge, a methodology for retrieval using emittance at different emission angles, θ, is developed here. The method contains four steps and takes advantage of emissometry. The method is firstly validated using simulation and then its feasibility is demonstrated by retrieving optical constants of a phase-change germanium-antimony-tellurium (Ge2Sb2Te5, GST) film. Emittance from samples at 100 °C, 200 °C, 300 °C, and 400 °C is measured at θ = 0°, 15°, and 30°. The spectral range of retrieval covers from 4 μm to 18 μm where thermal radiation dominates. The investigated film considers amorphous, face-centered cubic, and hexagonal close packed phases. The retrieved constants exhibit temperature and substrate independence, but they show a significant phase reliance.