Second Harmonic Generation in Exfoliated Few-Layer ReS₂

The second harmonic generation (SHG) is systematically investigated in mechanically exfoliated, few-layer ReS2 samples. The stacking orders are identified as (AA)(n)A and (AB(y))(n)A for (2n+1)-layer samples and (AA)(n) and (AB(y))(n) for 2n-layer samples. The layer structure A stands for that of a monolayer, and the layer structure B-y is mirror to A followed by a one-tenth unit cell translation along the mirror axis, that is, the b-axis of the crystal. Only the even-layer samples with (AB(y))(n) stacking order are SHG active. After carefully extracting the effective bulk susceptibility for SHG, it is found that the spectra are greatly enhanced as the fundamental photon energy matches exciton levels. The polarization dependence is strongly anisotropic, and each susceptibility component shows very different wavelength dependence. The results demonstrate that few-layer ReS2, with its unique SHG, can be considered as an ideal platform for fundamental research in anisotropic nonlinear optics.