  The ability to engineer metamaterials with tunable nonlinear optical
properties is crucial for nonlinear optics. Traditionally, metals have been
employed to enhance nonlinear optical interactions through field localization.
Here, inspired by the electronic properties of materials, we introduce and
demonstrate experimentally an asymmetric metal-semiconductor-metal (MSM)
metamaterial that exhibits a large and electronically tunable effective
second-order optical susceptibility (\c{hi}(2)). The induced \c{hi}(2)
originates from the interaction between the third-order optical susceptibility
of the semiconductor (\c{hi}(3)) with the engineered internal electric field
resulting from the two metals with dissimilar work function at its interfaces.
We demonstrate a five times larger second-harmonic intensity from the MSM
metamaterial, compared to contributions from its constituents with electrically
tunable nonlinear coefficient ranging from 2.8 to 15.6 pm/V.
