Crosstalk and supermode formation in cascades of magneto-plasmonic ring switches

Surface plasmon polariton (SPP) magneto-optical ring switches based on ferromagnetic-dielectric/metal/ferromagnetic-dielectric (IMI) resonators have recently been proposed as compact, magnetically reconfigurable building blocks for plasmonic circuitry. While the single-element response has been characterised in detail, the scalability of such switches into multi-element cascades, a prerequisite for any non-trivial integrated functionality, has so far received no quantitative attention. Here we develop a coupled-mode-theory (CMT) framework that describes a one-dimensional cascade of N magneto-plasmonic ring resonators side-coupled to a single SPP waveguide, including bus propagation, magnetisation-induced detuning, and near-field ring–ring hybridisation. We show that two rings spaced closer than the evanescent decay length Λ of the coupling integral hybridise into supermodes whose splitting reaches values comparable to or larger than one resonance linewidth. At the critical-coupling operating point demonstrated before, however, these supermodes remain spectrally unresolved and are masked inside a single, broadened transmission notch. As a counter-intuitive consequence, a naive cascade of identical switches degrades the magnetic switching contrast monotonically with N rather than enhancing it: the redistribution of supermodes restores transmission at ω 0 also under uniform magnetisation. Operated at fixed frequency, the same cascade behaves nonetheless as a magnetically programmable transmittance encoder with 2 N addressable magnetisation patterns. Our results identify hidden hybridisation as the dominant scalability bottleneck of magneto-plasmonic ring circuitry and indicate design strategies (under-coupling, off-resonance operation, alternating magnetisation patterns) to overcome it.