Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3

• Published in: Nature (London) Vol. 572; no. 7770; pp. 497 - 501
• Main Authors: Sun, Zeyuan, Yi, Yangfan, Song, Tiancheng, Clark, Genevieve, Huang, Bevin, Shan, Yuwei, Wu, Shuang, Huang, Di, Gao, Chunlei, Chen, Zhanghai, McGuire, Michael, Cao, Ting, Xiao, Di, Liu, Wei-Tao, Yao, Wang, Xu, Xiaodong, Wu, Shiwei
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 22-08-2019; Nature Publishing Group
• Subjects: 140/125; 639/301/357/1018; 639/766/119/997; 639/766/400/385; Antiferromagnetism; Chromium; Crystal structure; Crystallography; Electronic devices; Ferromagnetism; Humanities and Social Sciences; Interlayers; Letter; Magnetic fields; Magnetic permeability; magnetic properties and materials; Magnetism; Magnets; MATERIALS SCIENCE; Molybdenum; Molybdenum disulfide; multidisciplinary; nonlinear optics; Optical materials; Optics; Photons; Resonant frequencies; Science; Science (multidisciplinary); Second harmonic generation; Spectrum analysis; Symmetry; two-dimensional materials
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1445-3

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI
3
) has been shown to be a layered antiferromagnetic insulator in its few-layer form
1
, opening up opportunities for various functionalities
2
–
7
in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI
3
. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG
8
–
11
and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far
12
,
13
(for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI
3
is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying
C
2
h
crystallographic symmetry—and thus monoclinic stacking order—in bilayer CrI
3
, providing key structural information for the microscopic origin of layered antiferromagnetism
14
–
18
. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.
Large second-harmonic generation is observed in antiferromagnetic bilayers of CrI
3
, providing information about the microscopic origin of layered antiferromagnetism and motivating the use of two-dimensional magnets in nonlinear optical devices.