Direct Determination of Photonic Stopband Topological Character: A Framework based on Dispersion Measurements

TL;DR

This paper introduces a novel method to determine the absolute topological character of photonic bandgaps directly from dispersion measurements, overcoming limitations of previous bulk-boundary and reflection phase techniques.

Contribution

It proposes a dispersion-based approach using Kramers-Kronig relations and defines a new parameter, Differential Effective Mass, for classifying topological bandgaps in photonic crystals.

Findings

Dispersion measurements can qualitatively identify topological bandgap identities.

The Differential Effective Mass parameter effectively classifies bandgap topology.

The method simplifies experimental procedures for topological characterization.

Abstract

Ascertainment of photonic stopband absolute topological character requires information regarding the Bloch eigenfunction spatial distribution. Consequently, the experimental investigations predominantly restrict themselves to the bulk-boundary correspondence principle and the ensuing emergence of topological surface state. Although capable of establishing the equivalence or inequivalence of bandgaps, the determination of their absolute topological identity remains out of its purview. The alternate method of reflection phase-based identification also provides only contentious improvements owing to the measurement complexities pertaining to the interferometric setups. To circumvent these limitations, we resort to the Kramers-Kronig amplitude-phase causality considerations and propose an experimentally conducive method for bandgap topological character determination directly from the parametric reflectance measurements. Particularly, it has been demonstrated that in case of one-dimensional photonic crystals, polarization-resolved dispersion measurements suffice in qualitatively determining bandgap absolute topological identities. By invoking the translational invariance of the investigated samples, we also define a parameter Differential Effective Mass that encapsulates bandgap topological identities and engenders an experimentally discernible bandgap classifier.