  We present a study of the second-order nonlinear optical properties of
metal-based metamaterials. A hydrodynamic model for electronic response is
used, in which nonlinear surface contributions are expressed in terms of the
bulk polarization. The model is in good agreement with published experimental
results, and clarifies the mechanisms contributing to the nonlinear response.
In particular, we show that the reported enhancement of second-harmonic in
split-ring resonator based media is driven by the electric rather than the
magnetic properties of the structure.
