In a study published in Advanced Optical Materials, researchers from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) have systematically investigated the second harmonic generation (SHG) in mechanically exfoliated, few-layer ReS₂ samples which is strongly dependent on stacking order, layer number, fundamental photon energy, and light polarization.

ReS₂, a member of 2D transition metal dichalcogenides (TMD) family, has attracted considerable interest due to its anisotropic optical, electronic, vibrational and ferroelectric properties which arise from its distorted 1T crystal structure. And due to the special interlayer interaction, there exists different stacking orders. However, the understanding of the effect of stacking geometry on SHG is still incomplete and how the excitonic effects affect the optical nonlinearity for strongly anisotropic few-layer ReS₂ is not well investigated.

In this study, they found that the SHG signals can be observed only in (AB_y)n-stacked even-layer samples, where the atomic structure of the B_y layer is a mirror of that of the A layer with mirror planes perpendicular to the b-axis, followed by a one-tenth displacement along this axis; and SHG vanished in all A_n- and (AB_y)_nA-stacked samples.

In the SHG-active AB_y-stacked even-layers, the SHG intensities can be greatly enhanced when the photon energy matches with the exciton level, and bulk SHG susceptibility always shows the largest value for the bilayer. The light polarization dependence of SHG is extremely irregular and photon energy related which confirm the strong anisotropy of few-layer ReS₂.

These results demonstrate that few-layer ReS₂, with its unique SHG, can be considered as an ideal platform for fundamental research in anisotropic nonlinear optics.