  We experimentally demonstrate stimulated four-wave mixing in two linearly
uncoupled integrated Si$_3$N$_4$ micro-resonators. In our structure the
resonance combs of each resonator can be tuned independently, with the energy
transfer from one resonator to the other occurring in the presence of a
nonlinear interaction. This method allows flexible and efficient on-chip
control of the nonlinear interaction, and is readily applicable to other
third-order nonlinear phenomena.
