Atomic Scale van der Waals Nonlinear Optical Metamaterials

Material’s responses to light are governed by symmetry. For example, the nonvanishing second-order susceptibilities of materials in the 32 point groups have been tabulated and experimentally confirmed. Methods breaking symmetries at interfaces of bulk crystals introduce new electric-dipolar second-order susceptibilities that do not exist in the natural bulk materials, enabling nonlinear metamaterials composed of centrosymmetric dielectrics (Si and Al₂O₃ <i>etc</i>.) and metals (Au, Ag and Al <i>etc</i>.). However, the interfacial nonlinear responses of bulk crystals are inefficient due to the inherently low interface area-to-volume ratio and prevent further miniaturization. The advent of twisted bilayers has provided a new way to create a pristine crystalline interface between two monolayers with an exceptionally low volume fraction. Reconfigured electronic potential in twisted structures enables non-localized nonlinear responses on an atomic scale, promising atomic scale nonlinear optical metamaterials. Most current research focuses on nonlinear responses caused by a superposition of constituent monolayers upon adjusting twist angle or stacking more layers while neglecting interfacial nonlinear responses. Exploring emerging interfacial second-order susceptibilities of van der Waals lattices can bring a new degree of freedom in nonlinear optics, which can be applied to a broad range of nonlinear nanophotonic applications.<br/><br/>In this work, second-harmonic generation (SHG) measurements on ±30° twisted bilayer WS₂ unveil the effect of an interfacial second-order sheet susceptibility (χ⁽²⁾_{s, xyz}) that does not exist in natural WS₂. In these samples, the interfacial second-harmonic (SH) polarization orientation is solely determined by an interfacial twist orientation. Thus, controlling the twist orientation of trilayer WS₂ enables enhancing and suppressing the interfacial nonlinear response. Furthermore, we devised and demonstrated a twisted quadruple WS₂ with a dominant interfacial nonlinear response (χ⁽²⁾_{s, xyz} ≠ 0 and Σχ⁽²⁾_{s, yyy} ≈ 0), which is significantly modified from monolayer WS₂ (χ⁽²⁾_{s, xyz} = 0 and Σχ⁽²⁾_{s, yyy} ≠ 0). The suppression of Σχ⁽²⁾_{s, yyy} in the quad-WS₂ is attributed to a screw axis symmetry. As stacking two quad-WS₂ vertically (octa-WS₂), we observe the interfacial sheet susceptibility increases linearly with the sample thickness, indicating a coherent interference in the thin material. Remarkably, our results demonstrate a new avenue to engineer each component of second-order susceptibilities by twisting and stacking WS₂ layers.