Exceptional points in topological spoof surface plasmon polaritons for index sensing applications

Exceptional points (EPs) in non-Hermitian systems provide a powerful platform for controlling electromagnetic waves. Here, we experimentally investigate a passive reflection-zero EP transition in a microwave spoof-surface-plasmon-polariton meta-strip supporting two coupled topological interface states. The mutual coupling between the interface states is systematically reduced by increasing their spatial separation. Temporal coupled-mode theory shows that the relevant transition is associated with the near-coalescence of the complex zeros of the left-reflection coefficient and their eigenvectors, rather than with the coalescence of the physical scattering poles. In the strong-coupling regime, two transmission maxima follow the two real reflection zeros. As the coupling decreases across the transition, the reflection zeros leave the real-frequency axis, and only one transmission maximum remains observable. The merging of the transmission maxima therefore provides a real-frequency signature of the reflection-zero transition but is not used alone as evidence of an EP. A localized dielectric perturbation produces a resolvable spectral splitting, illustrating the refractive-index sensing potential of this microwave topological platform.