Nonlinear modal excitation of coherent plasmonic structures

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.