Dynamic electromagnetic structures, which vary in both space and time, enable unique operational regimes and effects unattainable in static systems due to modal orthogonality constraints. This paper presents a theoretical framework for intermodal energy transfer in time-varying plasmonic structures. By identifying a suitable mechanism for permittivity modulation, we develop a time-domain formalism to analyze the evolution of the dielectric polarization density in the system. Through a perturbative approach, we derive closed-form solutions that describe the energy transfer between a directly excited dipolar mode and a higher-order subradiant mode. We also demonstrate that the modal amplitudes reach a steady state under optimal modulation conditions, which maximize the amplitude of the high-order mode. Finally, we propose a coherent control strategy to enhance the conversion efficiency to higher-order modes.
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