Investigation of the Nonlinear Optical Frequency Conversion in Ultrathin Franckeite Heterostructures

Layered franckeite is a natural superlattice composed of two alternatinglayers of different compositions, SnS_2- and PbS-like. This createsincommensurability between the two species along the planes of the layers,resulting in spontaneous symmetry-break periodic ripples in the a-axisorientation. Nevertheless, natural franckeite heterostructure has shownpotential for optoelectronic applications mostly because it is a semiconductorwith 0.7 eV bandgap, air-stable, and can be easily exfoliated down to ultrathinthicknesses. Here, we demonstrate that few-layer franckeite shows a highlyanisotropic nonlinear optical response due to its lattice structure, whichallow for the identification of the ripple axis. Moreover, we find that thehighly anisotropic third-harmonic emission strongly varies with materialthickness. These features are further corroborated by a theoretical nonlinearsusceptibility model and the nonlinear transfer matrix method. Overall, ourfindings help to understand this material and propose a characterization methodthat could be used in other layered materials and heterostructures to assigntheir characteristic axes.