  A purely artificial mechanism for optical nonlinearity is proposed based on a
metamaterial route. The mechanism is derived from classical electromagnetic
interaction in a meta-molecule consisting of a cut-wire meta-atom nested within
a split-ring meta-atom. Induced by the localized magnetic field in the
split-ring meta-atom, the magnetic force drives an anharmonic oscillation of
free electrons in the cut-wire meta-atom, generating an intrinsically nonlinear
electromagnetic response. An explicit physical process of a second-order
nonlinear behavior is adequately described, which is perfectly demonstrated
with a series of numerical simulations. Instead of "borrowing" from natural
nonlinear materials, this novel mechanism of optical nonlinearity is
artificially dominated by the meta-molecule geometry and possesses
unprecedented design freedom, offering fascinating possibilities to the
research and application of nonlinear optics.
