Nonreciprocal devices that allow the light propagation in only one direction are indispensable in photonic circuits and emerging quantum technologies. Contemporary optical isolators and circulators, however, require large size or strong magnetic fields because of the general weakness of magnetic light-matter interactions, which hinders their integration into photonic circuits. Aiming at stronger magneto-optical couplings, a promising approach is to utilize nonlinear optical processes. Here, we demonstrate nonreciprocal magnetoelectric second harmonic generation (SHG) in CuB2O4. SHG transmission changes by almost 100% in a magnetic-field reversal of just 10 mT. The observed nonreciprocity results from an interference between the magnetic-dipole- and electric-dipole-type SHG. Even though the former is usually notoriously smaller than the latter, it is found that a resonantly enhanced magnetic-dipole-transition has a comparable amplitude as non-resonant electric-dipole-transition, leading to the near-perfect nonreciprocity. This mechanism could form one of the fundamental bases of nonreciprocity in multiferroics, which is transferable to a plethora of magnetoelectric systems to realize future nonreciprocal and nonlinear-optical devices.
A method to decrease the harmonic distortion in Mn-Zn ferrite/PZT and Ni-Zn ferrite/PZT layered composite rings exhibiting high magnetoelectric effects
Magneto-toroidal nonreciprocity of second harmonic generation
洛伦兹互易原理是一个基本概念，它支配着光在零磁场中的任何光学线性介质中的传播。在这里，我们在实验上证明了一种新的机制，在非线性光学由环矩驱动的互易性破缺。使用高分辨率飞秒光谱在磁电反铁磁CuB $_2$ O $_4$的光电子共振，我们表明，通过控制的相干源的二次谐波产生的环形自旋顺序，施加的磁场，和非中心对称的晶体结构的非线性干扰，我们诱导一个巨大的非互易性接近100%相反的磁场。基于磁对称性和晶体对称性的理论分析证实了实验结果。这些发现为电子和磁结构的非线性物理学开辟了新的自由度，并为未来在飞秒时间尺度上运行的非互易自旋光电子器件铺平了道路。
Multiferroic-like Quasiparticles in Ferroelectrics
WPI-AIMR, Tohoku University（东北大学世界顶尖科学中心）；Institute for Materials Research and CSIS, Tohoku University（东北大学材料研究所和CSIS）；Kavli Institute for Theoretical Sciences, University of the Chinese Academy of Sciences（中国科学院大学卡尔维理论科学研究所）
Magnetic Order and Symmetry in the 2D Semiconductor CrSBr
最近发现的二维（2D）磁铁提供了独特的机会，为实验探索低维磁学4和磁邻近效应，并为新的磁电，磁光和自旋电子器件的发展。这些进展需要具有不同磁性结构的2D材料以及有效的磁对称性探针，这是理解层内磁序和层间磁耦合的关键。在这里，我们应用二次谐波产生（SHG），一种对对称性破缺非常敏感的技术，来探测一种新的二维磁性半导体CrSBr的磁性结构。我们发现，CrSBr单层铁磁有序低于146 K，一个巨大的磁偶极子SHG效应的中心对称的2D结构的发现，使观察。在多层膜中，铁磁单层膜是反铁磁耦合的，Néel温度随着层数的减少而显著增加。CrSBr的磁结构包括与矩形晶胞共面对准的自旋，与具有面外力矩的原型2D六边形磁体CrI 3和Cr2 Ge 2 Te 6显著不同。此外，我们的二次谐波分析表明，铁磁单层和反铁磁双层的序参量分别是磁偶极子和磁环矩。这些发现确立了CrSBr作为一种令人兴奋的2D磁性半导体和SHG作为探测2D磁对称性的有力工具，为探索磁序和激子/电子性质之间的耦合以及磁环矩在广泛的应用中打开了大门。
