3D nonlinear optical metamaterials from twisted 2D van der Waals interfaces

Metamaterials for nonlinear optics are created by organizing structural units (meta-atoms) which are typically on the scale of about a hundred nanometers. However, truly altering atomic symmetry and enabling new nonlinear responses requires control at the atomic scale, down to a few angstroms. Here, we report the discovery of 3D nonlinear optical van der Waals (vdW) metamaterials realized by precise control of individual atomic-scale interfaces. We theoretically show and experimentally demonstrate that adding a screw axis symmetry up to a twisted eight-layer WS₂ stack entirely alters the allowed nonlinear susceptibility components, as new nonlinear susceptibility components of interfaces can be enabled while those of individual layers can be forbidden. We further show that the interfacial nonlinear responses that do not exist in natural WS₂ can be enhanced by increasing the number of constituent layers of the metamaterials. Our findings suggest a new approach to reconfiguring the allowed nonlinear susceptibility components of vdW materials via electronic wavefunction symmetry engineering.