Experimental Verification of Ill-defined Topologies and Energy Sinks in Electromagnetic Continua

TL;DR

This paper experimentally demonstrates that nonreciprocal photonic systems with ill-defined topologies can be regularized using a spatial cutoff, leading to energy sinks that abruptly halt wave energy flow.

Contribution

It provides the first experimental verification of ill-defined topologies in nonreciprocal systems and introduces a method to regularize topology using an air gap.

Findings

Insertion of an air gap regularizes the topology.

Ill-defined topologies can create energy sinks.

Energy flow can be abruptly halted in waveguides.

Abstract

In this article, it is experimentally verified that nonreciprocal photonic systems with a continuous translation symmetry may have an ill-defined topology. The topological classification of such systems is only feasible when the material response is regularized with a spatial-frequency cutoff. Here, we experimentally demonstrate that inserting a small air gap in between two materials may effectively imitate an idealized spatial cutoff that suppresses the nonreciprocal response for short wavelengths and regularizes the topology. Furthermore, it is experimentally verified that nonreciprocal systems with an ill-defined topology may be used to abruptly halt the energy flow in a unidirectional waveguide due to the violation of the bulk-edge correspondence. In particular, we report the formation of an energy sink that absorbs the incoming electromagnetic waves with a large field enhancement at the singularity.