Strain modulation of second harmonic generation in new tetrahedral transition metal dichalcogenide monolayers

Designing novel two-dimensional structures and precisely modulating their second harmonic generation (SHG) attributes are key to advancing nonlinear photonic technologies. In this work, through first-principles calculations, we propose a novel tetrahedral phase of transition metal dichalcogenides (TMDs) and validate its structural feasibility in a family of compounds, i.e., ZX₂ (Z = Ti, Zr, Hf; X = S, Se, Te). Cohesive energy and phonon dispersion calculations further demonstrate that eight of nine possible ZX₂ monolayers are dynamically stable. All the ZX₂ monolayers exhibit pronounced out-of-plane SHG with nonlinear susceptibility components reaching the order of 10² pm/V. Strain engineering imposes a profound influence on the SHG response of ZX₂ monolayers by reducing symmetry and modifying nonlinear susceptibility components. The redshift and significant enhancement of the prominent peak in SHG spectra are also revealed due to strain-induced charge redistribution and band gap reduction. Intriguingly, strain-driven nonlinear optical switching effects are realized in the ZX₂ monolayers, with a reversible switching of SHG component ordering under tensile and compressive strain. In such a case, the anisotropic SHG pattern transforms from fourfold to twofold symmetry under the strain. Our work demonstrates the efficacy of strain engineering in precisely enhancing SHG, paving the way for the integration of novel TMD structures into tunable and flexible nonlinear optical devices.