Nonreciprocal second-harmonic generation in few-layer chromium triiodide

It is of fundamental importance but challenging to simultaneously identify atomic and magnetic configurations of two-dimensional van der Waals materials. In this work, we show that the nonreciprocal second-harmonic generation (SHG) can be a powerful tool to answer this challenge. Despite the preserved lattice inversion symmetry, the interlayer antiferromagnetic order and spin-orbit coupling generate and enhance SHG in parity-time (PT)-symmetric bilayer chromium triiodide (CrI3). Importantly, the in-plane polarization-resolved SHG is sensitive to the subtly different interlayer structures that cannot be differentiated by linear optical spectra. Beyond bilayer, we further predict that the intensity and angle-resolved SHG can be employed to identify both interlayer atomic and magnetic configurations of trilayer CrI3. Our first-principles results agree with available measurements and show the potential of SHG as a noncontact approach to explore correlations between interlayer structures and magnetic orders of emerging ultrathin magnetic materials.