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二次谐波产生(SHG)是一种在中心反演(P)对称性被破坏的材料中,在光照下会产生频率加倍的光信号的效应,被广泛应用于制造激光器,设计各种光学器件以及探测材料的各种对称性和物理性质。P对称性通常由晶格结构破坏,会产生时间反演(T)操作下不变的SHG——crystal SHG; 更有趣的是,P对称性可以单独由材料的磁序破坏,从而产生对应的T操作下反号的SHG——MSHG。两者在T操作下(即翻转磁矩)可以实现干涉相长和干涉相消两种状态,从而改变SHG信号,根据体系对称性的不同产生丰富的光学现象,称为非线性磁光(NLMO)效应。然而,早期实验对NLMO效应的研究集中在体材料中,而他们的MSHG总是远小于crystal SHG,从而导致两者的干涉效应很弱,其应用受到极大限制。柳暗花明,最近的实验在二维磁性材料中观测到了巨大的MSHG,如双层反铁磁CrI3,比一些具有巨大crystal SHG的二维材料如MoS2还要大。
受此启发,来自中物院研究生院叶萌课题组、清华大学物理系徐勇/段文晖团队通过第一性原理计算和对称性分析研究了同时具有crystal SHG和MSHG的二维磁性材料,成功在其中揭示了一类巨大且可调控的NLMO效应。他们发现不同于以往实验研究的体材料,某些二维磁性材料,如三层CrI3和单层Cr2I3Br3,可在特定入射光频率处具有振幅相当的crystal SHG和MSHG,可以在磁矩翻转下产生最大化的干涉效应,使线偏振SHG偏振面旋转90°,打开或关闭圆偏振SHG光强,同时在固定磁构型时可以选择性导通某种手性的圆偏振光——即100%的SHG圆二色性。这些效应在单层材料中可以实现磁场调控的线偏振光偏振转换器、圆偏振光光开关和滤波器,在多层材料中还可以用于区分精细的磁构型。此外,作者还推导出了实现这些效应所需的SHG振幅和相位条件,并提出了利用二维材料的高度可调控性,通过调节层间距、自选轨道耦合强度、堆垛工程等方法来调控crystal SHG和MSHG的绝对和相对大小,达到两者振幅相当,从而广泛地实现这类巨大的NLMO效应。最后,他们还通过CrBr3和多层VSe2验证了这些调控方法的有效性和可迁移性,证明了这些巨大的NLMO效应是可调控的。
这项研究丰富了对NLMO效应的理解,增强了其应用,并提供了新的方法来调控不同类型SHG大小,为二维磁性材料的应用提供了新的可能性。
Fig. 1 | Concept of NLMO effects.
Fig. 2 | Atomic structures and SHG of the representative 2D magnets.
Fig. 3 | NLMO effects in representative 2D magnets.
Giant and controllable nonlinear magneto-optical effects in two-dimensional magnets
Dezhao Wu, Meng Ye, Haowei Chen, Yong Xu & Wenhui Duan
The interplay of polarization and magnetism in materials with light can create rich nonlinear magneto-optical (NLMO) effects, and the recent discovery of two-dimensional (2D) van der Waals magnets provides remarkable control over NLMO effects due to their superb tunability. Here, based on first-principles calculations, we reported giant NLMO effects in CrI3-based 2D magnets, including a dramatic change of second-harmonics generation (SHG) polarization direction (90°) and intensity (on/off switch) under magnetization reversal and a 100% SHG circular dichroism effect. We further revealed that these effects could not only be used to design ultra-thin multifunctional optical devices but also to detect subtle magnetic orderings. Remarkably, we analytically derived conditions to achieve giant NLMO effects and proposed general strategies to realize them in 2D magnets. Our work not only uncovers a series of intriguing NLMO phenomena but also paves the way for both fundamental research and device applications of ultra-thin NLMO materials.
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