Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI₃

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI₃) has been shown to be a layered antiferromagnetic insulator in its few-layer form¹, 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 CrI₃. 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 CrI₃ 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_2h crystallographic symmetry—and thus monoclinic stacking order—in bilayer CrI₃, 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.