Detection of time-reversal symmetry breaking via waveguide mode coupling*

Time reversal symmetry is central to the characterization of a wide variety of quantum materials as it places fundamental constraints on electronic properties. Breaking such symmetry often reflects the topological features of the underlying ground state, points to the emergence of internal magnetic fields and gives rise to Hall currents. Experimental probes which are sensitive to this symmetry breaking without relying on contacts are relatively rare, and, as a result, it is often challenging to unequivocally establish the presence and microscopic origin of time-reversal symmetry breaking. Here, we present an alternative approach to detect time-reversal symmetry breaking based on coupling between electromagnetic modes in a cross-waveguide geometry. Specifically, we consider a small dielectric sample at the intersection of the waveguides and show how the breaking of time-reversal symmetry manifests in the scattered field. We conclude by discussing various experimental platforms where this technique may be applied.