Nonlinear optical colloidal metacrystals

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.