  Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers
with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet,
chromium triiodide (CrI3), emerged as the first layered antiferromagnetic
insulator in its few-layer form, opening up ample opportunities for novel
device functionalities. Here, we discovered an emergent nonreciprocal second
order nonlinear optical effect in bilayer CrI3. The observed second harmonic
generation (SHG) is giant: several orders of magnitude larger than known
magnetization induced SHG and comparable to SHG in the best 2D nonlinear
optical materials studied so far (e.g. MoS2). We showed that while the parent
lattice of bilayer CrI3 is centrosymmetric and thus does not contribute to the
SHG signal, the observed nonreciprocal SHG originates purely from the layered
antiferromagnetic order, which breaks both spatial inversion and time reversal
symmetries. Furthermore, polarization-resolved measurements revealed the
underlying C2h symmetry, and thus monoclinic stacking order in CrI3 bilayers,
providing crucial structural information for the microscopic origin of layered
antiferromagnetism. Our results highlight SHG as a highly sensitive probe that
can reveal subtle magnetic order and open novel nonlinear and nonreciprocal
optical device possibilities based on 2D magnets.
