The ability to engineer metamaterials with tunable nonlinear optical properties is crucial for nonlinear optics. We theoretically and experimentally demonstrate a novel approach to create and/or enhance large second-order nonlinear optical susceptibilities (χ(2)) in electronic metamaterials consisting of dielectric-semiconductor-dielectric (DSD) multilayers. The generated fixed charges (Qf) at dielectric/semiconductor interfaces are exploited to engineer a non-zero built-in electric field within a semiconductor layer asymmetrically cladded with different dielectric layers that create opposite signs of Qf. The asymmetry of these charges extends the depletion region into the entire semiconductor layer and consequently, the induced high internal electric field interacts with the third-order nonlinear susceptibility (χ(3)) of the semiconductor resulting in an enhanced, prominent effective χ(2) in the bulk of asymmetric DSD metamaterials. We investigate this composite effect by studying free-space second-harmonic generation via Maker fringes analysis technique, simultaneously calculating the components of the effective χ(2) tensor in various DSD composite metamaterials. The highest component, χ(2)zzz, is 2 pm/V for the as-fabricated single period silicon dioxide (SiO2) / amorphous silicon (a-Si) / aluminum oxide (Al2O3) multilayer stack. The magnitude of χ(2)zzz is further enhanced to 8.5 pm/V after thermal annealing processes. Also, 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 (χ(2)). The induced χ(2) originates from the interaction between the third-order optical susceptibility of the semiconductor (χ(3)) with the engineered internal electric field resulting from the two metals possessing 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. Spatial patterning of one of the metals on the semiconductor demonstrates tunable nonlinear diffraction, paving the way for all-optical spatial signal processing with space-invariant and -variant nonlinear impulse response. Finally, the constituents of this nonlinear composite DSD and MSM metamaterials, and the deposition technique make its manufacturing compatible with CMOS process, enabling their application for chip-scale silicon photonic integrated circuits and free space applications.
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