  To enable new nonlinear responses, metamaterials 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 metamaterials
realized by the precise control and twist of individual 2D van der Waals
interfaces. Specifically, new nonlinear crystals are achieved by adding pseudo
screw symmetries to a multiple of 4-layer WS2 stacks (e.g. 4-layer, 8-layer,
etc). The nonlinear susceptibility of the resulting 3D crystal is fundamentally
different from natural WS2. Furthermore, we show that the magnitude of the
newly enabled nonlinearity is enhanced by controlling the number of interfaces
and the excitation wavelength. Our findings suggest a new approach to
redesigning intrinsic nonlinearity in artificial atomic configurations,
scalable from a few nanometer-thick unit cells to bulk materials.
