Nonlinear Optical Colloidal Three-Dimensional Metacrystals

Atomic and molecular structure inversion symmetry breaking in naturally occurring crystals dictate their physical properties including nonlinear optical effects, piezo-/ferroelectricity, and nonreciprocal charge transport behavior. With metamaterials composed of nanoscale building blocks (<i>i.e.</i>, meta-atoms), the spatial inversion symmetry violation on planar surfaces leads to spin-controlled photonics as well as nonlinear optical metasurfaces. While low-symmetry three dimensional (3D) metacrystals can be synthesized, limitations in long-range order and control over resulting symmetries inhibits the investigation of the symmetry-property relationships in these systems. Here we present a DNA-mediated gold nanoparticle assembly approach to create 3D colloidal crystals which can be designed to deliberately access high- or low-symmetry phases. By manipulating particle shape, size, and DNA design, we effectively tune the crystal symmetries of the superlattices in a controllable manner. Access to these different symmetries and facile transitions among them enable us to explore the symmetry-dictated functionalities of these colloidal crystals. Further, we investigate how the resulting crystal symmetry relates to their nonlinear optical interactions and identify that the non-centrosymmetric crystal functions as an effective nonlinear optical metacrystal, where second harmonic generation (SHG) arises from the asymmetrical distribution of the local electric field around the close-packed plasmonic nanoparticles. Moreover, this non-centrosymmetric colloidal metacrystal represents the first 3D nonlinear optical metamaterial developed through a bottom-up approach and exhibits a high maximum SHG conversion efficiency, notably surpassing the efficiencies observed in the majority of plasmonic 2D metasurfaces by two orders of magnitude.