  We proposed a mechanism to generate giant anisotropic second harmonic
nonlinear response via double resonance effect, achieved through band nesting
via electronic bandstructure engineering. The ideal band setup would be a
triplet of nested bands separated by the fundamental resonance energy,
$\hbar\omega$. We demonstrate theoretically that the proposed phenomenon can be
realized in bilayer SnS by band tuning with perpendicular electrical bias,
which maximizes the second harmonic susceptibility by several orders of
magnitude. Moreover, the tunability of the polarization anisotropy can be
useful for realizing novel polarization-sensitive devices.
