Microwave seminar | 17 July 2023

The importance of quantized invariants is paramount in topological studies, as they not only characterize the global topology but keeping track of their evolution also allows us to capture the exotic phase transition phenomena. However, when it comes to the experimental determination of topological invariants predominantly, we rely on the bulk-boundary correspondence principle (an improvident approach in bosonic systems) and the limited information it provides. To break free from these limitations we have devised two new frameworks for the experimental determination of topological character: In the first method, topological equivalence/inequivalence of photonic bandgap has been ascertained by examining the ellipsometric phase shifts. Here, a direct correspondence between the appearance of topologically robust phase singularities in the angular dispersion space and the underlying bulk-band topology has been established. Moving a step further, we crossed the limitations of interferometric/ellipsometric measurements and demonstrated a new approach for experimental determination of bandgap absolute topological character, solely based on intensity signatures. Crucially, going beyond the mere establishment of equivalence/inequivalence of bandgaps or a phase transition phenomenon, this framework assigns a differential effective mass parameter to the bandgaps that can serve as an experimental marker for the associated topological identities.

Main paper/arXiv, related to the seminar, and other references

(1) Nitish Kumar Gupta, Mukesh Kumar, Anjani Kumar Tiwari, Sudipta Sarkar Pal, Harshawardhan Wanare, and S. Anantha Ramakrishna, "Spectroscopic ellipsometry-based investigations into the scattering characteristics of topologically distinct photonic stopbands." Applied Physics Letters 121, no. 26 (2022): 261103.

(2) Nitish Kumar Gupta, Sapireddy Srinivasu, Mukesh Kumar, Anjani Kumar Tiwari, Sudipta Sarkar Pal, Harshawardhan Wanare, and S. Anantha Ramakrishna, "Direct Determination of Photonic Stopband Topological Character: A Framework based on Dispersion Measurements" (under review).

(3) Nitish Kumar Gupta, Sapireddy Srinivasu, Mukesh Kumar, Anjani Kumar Tiwari, Sudipta Sarkar Pal, Harshawardhan Wanare, and S. Anantha Ramakrishna, “Experimental Demonstration of Topological Zero Modes in Nanophotonics” (about to submit).