Three-dimensional nonlinear optical materials from twisted two-dimensional van der Waals interfaces

To enable new nonlinear responses, metamaterials are created by organizing structural units (meta-atoms), which are typically on the scale of about a hundred nanometres. However, truly altering the atomic symmetry and enabling new nonlinear responses requires control at the atomic scale, down to a few ångströms. Here we report three-dimensional nonlinear optical materials realized by the precise control and twist of individual two-dimensional van der Waals interfaces. Specifically, new nonlinear crystals are achieved by adding pseudo-screw symmetries to a multiple of four-layer WS₂ stacks (for example, four layer, eight layer and so on). Nonlinear susceptibility and circular selectivity of the resulting three-dimensional crystals are fundamentally different from natural WS₂, demonstrating a microscopic analogue to the fabrication of metamaterials with unique optical properties. 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 redesign the intrinsic nonlinearity in artificial atomic configurations, scalable from a few-nanometre-thick unit cells to bulk materials.