Spectroscopic ellipsometry-based investigations into the scattering characteristics of topologically distinct photonic stopbands

Abstract

Topological band theory provides a framework to establish the equivalence/inequivalence of bandgaps in photonic topological insulators. However, experimental discernment of bandgap topological characteristics encounters inherent measurement complexities, particularly beyond the terahertz frequencies. To surmount this difficulty, we resort to the prolific optical technique of spectroscopic ellipsometry and carry out detailed experimental examination of attributes of one-dimensional photonic crystal stopbands and, in consequence, identify an appropriate classifier of the implicit topological characteristics. It is found that governed by the bulk topology, the band edge locations in the dispersion diagram provide a conditional site for the appearance of zeros of a complex reflection ratio. This leads to a selective appearance of topologically robust phase singularities with integer (unity positive) topological charge. We demonstrate that the presence of these phase singularities on either the blue or the red band edges of the stopbands provides us with an experimental marker of their distinctive topological characteristics.