Large sliding regulation in van der waals layered nonlinear optical ternary chalcogenides 

Qingchen Wu, Lei Kang, Jian Wu & Zheshuai Lin 

npj Computational Materials  9: 171 (2023).

编辑概述：范德瓦尔斯层状材料中的层间滑动机制与非线性光学特性设计

层间滑动为调节范德瓦尔斯（vdW）层状材料的铁电性和非线性光学（NLO）特性开辟了新途径，特别是在三元层状硫属化合物如Cu₂MoS₄中，通过滑动可实现二次谐波产生（SHG）效应的显著调节。这一发现对于光电子和光子学领域的应用，如宽光谱光频转换器和传感器等，具有划时代的意义。目前的研究焦点在于寻找具有敏感SHG响应和可识别调制机制的vdW层状材料。由中国科学院理化技术研究所功能晶体与激光技术重点实验室康雷研究员和林哲帅研究员领导的团队，系统研究了A₂MZ₄型三元范德瓦尔斯层状硫属化合物的可调二次谐波产生效应。该效应可以通过P/I相变进行调节，这种相变特征是层间以（1/2, 1/2, 0）的位移发生变化，但不改变非中心对称结构。在所有研究的34种材料中，Cd₂GeSe₄在SHG强度上有极大的调节比率（高和低SHG状态之间的变化超过36倍），使其成为实际应用中可调甚至可开关非线性光学性质的有力竞争者。理论分析表明，在层状Cd₂GeSe₄中，占据态的SHG活性能带可以被划分为3个区域，而SHG强度变化主要发生在以Se 4p轨道为主的I相近边区。然后作者发现，局域化Se 4p轨道上不同的层间Se…Se电荷耦合虽然是由弱vdW相互作用引起的，但对于Cd₂GeSe₄两种堆叠类型的强SHG偏差至关重要。AB型堆叠的I相更有利于层间Se…Se耦合的形成，削弱了Se 4p轨道的局域化，从而影响了它们对SHG的贡献。这种调节模型还可以进一步扩展到具有不同元素组成的其他取代型A₂MZ₄型层状硫属化合物，为通过调节层间电荷重分布来调整范德瓦尔斯层状材料的光学非线性引入了一种机制。 

Editorial Summary: Interlayer Sliding Mechanism and Nonlinear Optical Property Design in Van der Waals Layered Materials

Interlayer sliding opens new avenues for tuning the ferroelectric and nonlinear optical (NLO) properties of van der Waals (vdW) layered materials, especially in ternary layered sulfur genus compounds such as Cu₂MoS₄, where significant tuning of the second-harmonic generation (SHG) effect can be achieved by sliding. This discovery is epoch-making for applications in optoelectronics and photonics, such as broad-spectrum optical frequency converters and sensors. Current research focuses on finding vdW layered materials with sensitive SHG response and recognizable modulation mechanisms. A team lead by Prof. Lei Kang （康雷研究员） and Prof. Zheshuai Lin （林哲帅研究员） from Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,systematically investigated the tunable SHG effect of A₂MZ₄-type ternary van der Waals layered chalcogenides which can be regulated by P/I phase change featuring a (1/2, 1/2, 0) interlayer shift without changing the non-centrosymmetric structure. Among all 34 investigated materials, Cd₂GeSe₄ has an extremely large regulation ratio in SHG strength (>36 times between high- and low-SHG states), making it a strong contender for practical materials with tunable and even switchable NLO properties. The theoretical analysis proposes that the occupied SHG-active bands in layered Cd₂GeSe₄ can be divided into 3 regions, and the SHG strength change is rooted in the near-edge region I dominated by Se 4p orbitals. The authors then find that the different interlayer Se…Se charge coupling on localized Se 4p orbitals, although resulting from the weak vdW interaction, is crucial for the strong SHG deviation in the two stacking types of Cd₂GeSe₄. The AB- stacked I-phase is more favorable for the formation of interlayer Se…Se coupling, weakening the localization of Se 4p orbitals and consequently influencing their SHG contribution. Such a regulation model can further be extended to other substituted A₂MZ₄-type layered chalcogenides with different elemental compositions, introducing a mechanism to adjust optical nonlinearity for vdW layered materials by tuning interlayer charge redistribution.