  Plasmonic metasurfaces are promising as enablers of nanoscale nonlinear
optics and flat nonlinear optical components. Nonlinear optical responses of
such metasurfaces are determined by the nonlinear optical properties of
individual nanostructured plasmonic meta-atoms, which are the building blocks
of the metasurfaces. Unfortunately, no simple methods exist to determine the
nonlinear coefficients (hyperpolarizabilities) of the meta-atoms hindering
designing of nonlinear metasurfaces. Here, we develop the equivalent RLC
circuit model of such meta-atoms to estimate their second-order nonlinear
optical parameter i.e. the first-order hyperpolarizability in the optical
spectral range. In parallel, we extract from second-harmonic generation
experiments the spectrum of the 1st-order hyperpolarizabilities of individual
meta-atoms consisting of asymmetrically shaped (elongated) plasmonic
nanoprisms. Moreover, we verify our results using nonlinear hydrodynamic-FDTD
and with calculations based on nonlinear scattering theory. All three
approaches: analytical, experimental, and computational, yield results that
agree very well. Our empirical RLC model can thus be used as a simple tool to
enable efficient design of nonlinear plasmonic metasurfaces.
