element in (c) has $a$ = 13 nm and $c$ = 50 nm.[]{data-label="fig:toymodel"}](Fig_toymodel.png)

First, electromagnetically coupled two plasmonic nanorods are illustrated using MST (Fig. \[fig:toymodel\]a and b), where only $(\alpha_{e1}^{e1})_{x}^{x}$ component is considered in their $\boldsymbol{\alpha}$-tensors. Such closely situated plasmonic particles are strongly coupled, resulting strong spectral resonance shift. Spectral red-shift (Fig. \[fig:toymodel\]a) and blue-shift (Fig. \[fig:toymodel\]b) can be reconstructed depending on the configuration of the coupled nanorods. This phenomena can be intuitively interpreted by plasmon hybridization theory (PHT) [@Nordlander2004]; the induced charge density configuration in Fig. \[fig:toymodel\]a becomes stable by the hybridization redshifting the resonance, whereas the configuration in Fig. \[fig:toymodel\]b becomes unstable blue-shifting the resonance. However, PHT is based on the quasistatic approximation, so quantitative analysis is difficult for large, complicated systems. Another widely used theoretical framework for interpreting coupled optical systems is the coupled mode theory (CMT), which approximates the scattering objects as harmonic oscillators that are coupled to each other. However, CMT relies on fitting procedure to retrieve the relevant parameters and requires experimental or simulated results to begin with, so the CMT cannot be used to provide new information. In addition, it is of question whether the fitted parameters from the simple coupled harmonic oscillators 