Intermodal excitation in time-varying plasmonic structures

Space and time-varying electromagnetic structures give access to regimes of operation and effects that ordinarily do not occur in their time-invariant counterparts due to modal orthogonality constraints. Here, we present the theory of intermodal energy transfer in time-varying plasmonic structures. After identifying a suitable physical mechanism of permittivity modulation, we introduce an appropriate time-domain formalism to study the evolution of the dielectric polarization density in the system. Using a perturbative approach, we obtain closed-form solutions describing the intermodal energy transfer between a directly excited dipolar mode and a higher order subradiant mode. We further show that the modal amplitudes reach a steady state and determine the optimal modulation conditions that maximize the amplitude of the high-order mode. Finally, we identify a coherent control strategy to enhance the conversion efficiency to higher order modes.