Unidirectional Maxwellian Spin Waves

Abstract: We develop a unified perspective of unidirectional topological edge waves in non-reciprocal media. We focus on the inherent role of photonic spin in non-reciprocal gyroelectric media, ie. magnetized metals or magnetized insulators. We first review the concept of a Maxwell Hamiltonian in non-reciprocal media, which immediately reveals that the gyrotropic coefficient behaves as a photon mass in two dimensions. Similar to the Dirac mass, this photonic mass opens bandgaps in the energy dispersion of bulk propagating waves. Within these bulk photonic bandgaps, three distinct classes of Maxwellian edge waves exist - each arising from subtle differences in boundary conditions. On one hand, the edge wave solutions are rigorous photonic analogs of Jackiw-Rebbi electronic edge states. On the other hand, for the exact same system, they can be high frequency photonic counterparts of the integer quantum Hall effect, familiar at zero frequency. Our Hamiltonian approach also predicts the existence of a third distinct class of Maxwellian edge wave exhibiting topological protection. The Maxwellian edge state in this unique \textit{quantum gyroelectric phase of matter} necessarily requires a sign change in gyrotropy arising from non-locality (spatial dispersion). A signature property of these topological electromagnetic edge states is that they are oblivious to the contacting medium, ie. they occur at the interface of the quantum gyroelectric phase and any medium (even vacuum). Furthermore, the Maxwellian spin waves exhibit photonic spin-1 quantization in exact analogy with their supersymmetric spin-\sfrac{1}{2} counterparts. The goal of this paper is to discuss these three foundational classes of edge waves in a unified perspective while providing in-depth derivations, taking into account non-locality and various boundary conditions.