  We investigate Second Harmonic Generation from individual silicon nanowires
and study the influence of resonant optical modes on the far-field nonlinear
emission. We find that the polarization of the Second Harmonic has a
size-dependent behavior and explain this phenomenon by a combination of
different surface and bulk nonlinear susceptibility contributions. We show that
the Second Harmonic Generation has an entirely different origin, depending on
whether the incident illumination is polarized parallel or perpendicularly to
the nanowire axis. The results open perspectives for further geometry-based
studies on the origin of Second Harmonic Generation in nanostructures of
high-index centrosymmetric semiconductors.
