Probing Noncentrosymmetric 2D Materials by Fourier Space Second Harmonic Imaging

The controlled assembly of twisted 2D structures requires precise determination of the crystal orientation of their component layers. In the established procedure, the second-harmonic generation (SHG) intensity of a noncentrosymmetric layer is recorded while rotating the polarization of both the incident laser field and detected SHG, which can be time-consuming and tedious. Here, we demonstrate that the crystal orientation of transition metal dichalcogenides and hexagonal boron nitride can be directly determined by recording SHG images generated by tightly focused laser beams in Fourier space. Using an azimuthally polarized laser beam, the SHG image distinctly reflects the hexagonal structure of the crystal lattice, revealing its orientation quickly and accurately. This technique could significantly impact the field of twistronics, which studies the effects of the relative angle between the layers of a stacked 2D structure, as well as advances the nanofabrication of 2D materials.