In this study, we investigated the plasmon resonances of split-ring resonators (SRRs) numerically at incident angles of 0 and 45° under illumination with linearly and circularly polarized waves. At 45° incidence, perpendicular polarized waves excited distinct odd plasmon modes; the difference in the reflections of right and left circularly polarized incident radiation was very large. From simulated near field plots, we found that the parallelism of the incident electric field and the induced plasmon current was the key factor affecting excitation. We propose the use of a parallelism factor (P-factor), based on a standing-wave approach, to characterize the ability of incident fields to excite multiple plasmon resonance currents. The mechanism of the field and current parallelism can explain the resonance behavior of SRRs when considering the polarization state, incident angle, and geometry of the SRR.
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