Broken symmetries induce strong even-order nonlinear optical responses in materials and at interfaces. Unlike conventional covalently bonded nonlinear crystals, van der Waals (vdW) heterostructures feature layers that can be stacked at arbitrary angles, giving complete control over the presence or lack of inversion symmetry at a crystal interface. Here, we report highly tunable second harmonic generation (SHG) from nanomechanically rotatable stacks of bulk hexagonal boron nitride (BN) crystals and introduce the term twistoptics to describe studies of optical properties in twistable vdW systems. By suppressing residual bulk effects, we observe SHG intensity modulated by a factor of more than 50, and polarization patterns determined by moiré interface symmetry. Last, we demonstrate greatly enhanced conversion efficiency in vdW vertical superlattice structures with multiple symmetry-broken interfaces. Our study paves the way for compact twistoptics architectures aimed at efficient tunable frequency conversion and demonstrates SHG as a robust probe of buried vdW interfaces.

对称性的破坏会在材料和界面处引起强烈的偶数阶非线性光学响应。与常规的共价键合非线性晶体不同，范德华（vdW）异质结构的特征在于可以以任意角度堆叠的层，从而完全控制了晶体界面处是否存在反转对称性。在这里，我们从块状六方氮化硼（BN）晶体的纳米机械旋转堆栈中报告了高度可调的二次谐波（SHG），并引入了术语扭曲光学来描述可扭曲vdW系统中光学性质的研究。通过抑制残留的体积效应，我们观察到SHG强度被调制超过50倍，并且偏振图案由莫尔界面对称性确定。最后的，我们证明了具有多个对称断开界面的vdW垂直超晶格结构中的转换效率大大提高。我们的研究为紧凑的光学光学体系结构铺平了道路，该体系结构旨在实现有效的可调频率转换，并证明了SHG作为埋入式vdW接口的可靠探测器。