Manipulating Anisotropy of Second Harmonic Generation in NbOI ₂ via Precise Strain Engineering

Abstract 2D in‐plane ferroelectric niobium oxide diiodide (NbOI 2 ) shows significant second harmonic generation (SHG) responses. While strain engineering is known to enhance its SHG intensity by increasing the structural polarity, the specific contributions from strain orientation in comparison with strain magnitude remain unexplored. Here, NbOI 2 SHG responses on both strain magnitude and orientation are investigated, and its nonlinear susceptibility tensor modulations quantitatively. Artificial strain engineering with precise orientations has been realized by fabricating grayscale sinusoidal surfaces using thermal scanning probe lithography. By applying 11 distinct strain orientations to NbOI 2 , the strain vector is successfully correlated with the NbOI 2 nonlinear susceptibility tensor quantitatively. In addition, a considerable SHG enhancement, ≈19‐fold, is realized by applying a uniaxial strain of only ≈0.1% along the polar axis of NbOI 2 . Finally, SHG anisotropy has been manipulated by aligning the strain orientation more closely with the polar axis of NbOI 2 . The findings unveil the intrinsic relationship between precise strain engineering and 2D ferroelectrics, paving the way for advanced nonlinear optoelectronic devices by manipulating structural anisotropy.