A Mutual Coupling Theory for Arbitrarily Distributed Trimeric Localized Surface Plasmonic Particles

Coupled mode theory (CMT) is widely used to analyze the coupling relationship of plasmons and metasurfaces. However, some details involving coupled structures remain obscure. Herein, the coefficients in CMT are properly defined to solve the problems of inaccurate fitting from two particles to three or more. The proposed improved coupled mode theory (i‐CMT) can perfectly express both the amplitude and phase of transmission curve of dimeric and trimeric mutual coupled systems. A surface plasmonic structure containing multiple metallic spiral structures is fabricated to experimentally verify the accuracy of the i‐CMT. The experimental results are consistent with the theoretical and simulated values, proving the feasibility of using the i‐CMT to analyze and design asymmetric coupled devices. Moreover, the application of the i‐CMT confirms the presence of additional coupling terms in three or more coupled objects, making the calculation of eigenfrequency more accurate. Coupled mode theory is able to account for the coupling of electromagnetic waves on multiple adjacent metal spiral structures. Related simulations, experiments, and calculations also prove this theory. Therefore, the amplitude and bandwidth of resonance can be adjusted by structural and material parameters. It also provides a method to verify the existence of additional coupling terms between the three couplings.