Analytic ellipsometric measurement for materials under bulk encapsulation

Accurate measurement of the dielectric functions of emerging optical materials is of great importance for advancements in solid-state physics. However, it is rather challenging since most materials are highly active in ambient conditions, which makes in-situ measurements tough. Here, we report an analytical ellipsometry method (AEM) accessible in ambient conditions for measuring the dielectric functions of chemically reactive materials under bulk encapsulation. Taking the highly pursued low-loss plasmonic materials, such as sodium films, as an example, the effectiveness and measuring errors of the proposed AEM have been systematically demonstrated. This verifies AEM's superiority in overcoming the limitations of traditional spectroscopic ellipsometry methodologies, which include complex multi-parameter fitting procedures and the issue of potentially unphysical results, especially in newly developed low-loss materials. Our results will provide a generalized and convenient ellipsometric measurement strategy for sensitive materials under bulk encapsulation.