from an electrostatic polarizability derived from a single resonant RLC equation of motion. Therefore we take a common resonant frequency dependence out of the tensor elements, writing $$\boldsymbol{\alpha}_{SRR}^{\mathrm{static}}=\alpha(\omega)
 \left(
 \begin{array}{c c c c c}
 \eta_{E} & 0 & ... & 0 &  i\eta_{C}\\
 0 &  &  &  & 0\\
\vdots &  & \ddots &  & \vdots\\
0 &  &  &  & 0\\
 -i\eta_{C} & 0 & ... & 0 &\eta_{H}
 \end{array}
\right),\label{Eq:SRRpolarizsimple}$$ where $\eta_E$, $\eta_C$ and $\eta_H$ are constant and $\alpha(\omega)$ is a Lorentzian prefactor $$\alpha(\omega)=\frac{\omega_0^2
V}{\omega_0^2-\omega^2-i\omega\gamma}. \label{Eq:prefactorSRR}$$ Here, $\omega_0$ is the SRR resonance frequency $\omega_0\approx
\frac{1}{\sqrt{LC}}$, $\gamma$ is the damping rate due to the Ohmic loss of gold and $V$ is the physical particle volume. As in the plasmonic case, this approximation is coined ‘quasi-static’, as it does contain frequency $\omega$, but does not contain the velocity of light $c$. The polarizability obtained from the quasi-static polarizability once the radiation damping term is added (section \[magnetoelectricsection\] Eq. (\[Eq:TensorRadiationDamping\])) is called ‘dynamic polarizability’. In this formulation, all the frequency dependence, and the units of $\boldsymbol{\alpha}_{SRR}$ are contained in $\alpha(\omega)$. The parameters $\eta_E$, $\eta_H$ and $\eta_C$ are dimensionless. For a lossless split ring $\eta_E$, $\eta_H$ and $\eta_C$ are all real. We 