  We experimentally explore the fingerprint of the microscopic electron
dynamics in second-order harmonic generation (SHG). It is shown that the
interbond electron hopping induces a novel source of nonlinear polarization and
plays an important role even when the driving laser intensity is 2 orders of
magnitude lower than the characteristic atomic field. Our model predicts
anomalous anisotropic structures of the SHG yield contributed by the interbond
electron hopping, which is identified in our experiments with ZnO crystals.
Moreover, a generalized second-order susceptibility with an explicit form is
proposed, which provides a unified description in both the weak and strong
field regimes. Our work reveals the nonlinear responses of materials at the
electron scale and extends the nonlinear optics to a previously unexplored
regime, where the nonlinearity related to the interbond electron hopping
becomes dominant. It paves the way for realizing controllable nonlinearity on
an ultrafast time scale.
