Giant Nonlinear Optical Response via Coherent Stacking of In‐Plane Ferroelectric Layers

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in‐plane ferroelectric material that exhibits a giant nonlinear optical effect, group‐IV monochalcogenide SnSe, is reported. Nanometer‐scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical‐vapor‐deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second‐harmonic fields and the as‐resulted second‐harmonic generation was observed to be 100 times more intense than monolayer WS2. This work demonstrates in‐plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on‐chip nonlinear optical components and nonvolatile memory devices. Ferroelectric materials are great candidates for nonlinear optics and electro‐optic modulators. A giant second‐harmonic generation effect is reported in physical vapor‐deposited ferroelectric material, SnSe. Nanoscale in‐plane ferroelectric domains are revealed, those with ferroelectric stacking exhibit ≈100 times higher nonlinear optical efficiency than monolayer TMDs, due to a parallel stacking structure where nonlinear dipoles in each vdW ferroelectric layer add constructively.