Yao Kaiyuan, Finney Nathan R, Zhang Jin, Moore Samuel L, Xian Lede, Tancogne-Dejean Nicolas, Liu Fang, Ardelean Jenny, Xu Xinyi, Halbertal Dorri, Watanabe K, Taniguchi T, Ochoa Hector, Asenjo-Garcia Ana, Zhu Xiaoyang, Basov D N, Rubio Angel, Dean Cory R, Hone James, Schuck P James
Department of Mechanical Engineering, Columbia University, New York, NY 10027, USA.
Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany.
Sci Adv. 2021 Mar 3;7(10). doi: 10.1126/sciadv.abe8691. Print 2021 Mar.
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.
对称性破缺会在材料及界面处引发强烈的偶数阶非线性光学响应。与传统的共价键合非线性晶体不同，范德华（vdW）异质结构的特点是其层间可以以任意角度堆叠，从而能够完全控制晶体界面处是否存在反演对称性。在此，我们报道了来自体相六方氮化硼（BN）晶体的纳米机械可旋转堆叠结构的高度可调谐二次谐波产生（SHG），并引入“扭转光学”一词来描述对可扭曲范德华系统光学性质的研究。通过抑制残余的体相效应，我们观察到SHG强度调制因子超过50，以及由莫尔界面对称性决定的偏振图案。最后，我们展示了在具有多个对称性破缺界面的范德华垂直超晶格结构中大大提高的转换效率。我们的研究为旨在实现高效可调谐频率转换的紧凑型扭转光学架构铺平了道路，并证明了SHG是探测埋藏范德华界面的有力探针。
