Refractive index inversion method for metals based on circular polarization detection of light

Abstract

Among many optical detection methods, the polarization detection method of light has been proven to be capable of providing outstanding, even unique sometimes, information of targets under test, this includes detecting objects that are difficult to be distinguished by the light intensity or spectrum information alone. However, as for materials identification, the current polarization detection method which usually uses linearly polarized light can only roughly distinguish metallic materials from non-metallic materials but cannot further differentiate among types of metallic materials. Note that the phase of a linearly polarized light may change after being reflected by a metal due to its complex refractive index, and the circular polarization information of the reflected light can be used to sense the phase change property of the metal, therefore, different types of metallic materials can be identified according to their characteristic circular polarization information due to their diverse complex refractive indices. In this paper, based on the circular polarization detection of light, a complex refractive index inversion method for metals is proposed accordingly, and experimental measurements and fitting estimations are carried out for different types of metallic materials using a Bidirectional Reflectance Distribution Function (BRDF) measurement equipment. The experimental results show that the method can accurately invert the complex refractive indices of different metals and distinguish different types of metal samples. Compared to the Mueller-matrix-based measurement method, the proposed method is computationally simple, needless of measuring a large number of parameters and performing complex calculations, it only needs to invert the results of twice measurements. In addition, it also has the advantages of high sensitivity, good stability and intuitive algorithm, which are valuable for identification applications of metallic materials.