Atomic and molecular structure inversion symmetry breaking in naturally occurring crystals dictate their physical properties including nonlinear optical (NLO) effects, piezo- or ferroelectricity, and non-reciprocal charge transport behaviour. With metamaterials composed of nanoscale building blocks (that is, meta-atoms), the spatial inversion symmetry violation on planar surfaces leads to spin-controlled photonics as well as NLO metasurfaces. Synthetically, low-symmetry 3D metacrystals can be synthesized, but NLO behaviour has not been identified so far (for example, harmonic generations). Herein we show how DNA-mediated assembly of octahedron-shaped plasmonic gold nanocrystals can be used to design and deliberately synthesize non-centrosymmetric and centrosymmetric colloidal crystals. Importantly, while the centrosymmetric structures do not exhibit substantial second-harmonic generation, the non-centrosymmetric crystals do—a consequence of the asymmetric distribution of localized electric fields in plasmonic hotspots. Moreover, this non-centrosymmetric NLO metacrystal represents a 3D NLO metamaterial being developed via a bottom-up approach, exhibiting a maximum second-harmonic generation conversion efficiency of 10⁻⁹ to surpass the efficiencies observed in the majority of plasmonic 2D metasurfaces. Finally, the DNA-loading density on the particle building blocks can be used to toggle between the centrosymmetric and non-centrosymmetric phases.

天然晶体中的原子和分子结构反转对称性破坏决定了它们的物理特性，包括非线性光学 （NLO） 效应、压电或铁电性以及非互易电荷传输行为。对于由纳米级构建块（即超原子）组成的超材料，平面上的空间反转对称性违规导致了自旋控制光子学和 NLO 超表面。在合成方面，可以合成低对称性 3D 超晶，但到目前为止尚未确定 NLO 行为（例如，谐波产生）。在此，我们展示了如何使用 DNA 介导的八面体形等离子体金纳米晶体组装来设计和有意合成非中心对称和中心对称胶体晶体。重要的是，虽然中心对称结构没有表现出实质性的二次谐波产生，但非中心对称晶体却表现出——这是等离子体热点中局域电场不对称分布的结果。此外，这种非中心对称 NLO 超晶代表了通过自下而上的方法开发的 3D NLO 超材料，其最大二次谐波产生转换效率为 ^10-9，超过了在大多数等离子体 2D 超表面中观察到的效率。最后，颗粒构建单元上的 DNA 负载密度可用于在中心对称和非中心对称相之间切换。