Remarkably enhanced second harmonic generation in Janus HfXY (X, Y = O, S, Se, Te; X ≠ Y) monolayers

Designing new two-dimensional (2D) materials with enhanced second harmonic generation (SHG) and uncovering the underlying mechanisms is crucial for enabling high-performance nonlinear photonics. In this work, we design a new Janus structure HfXY (X, Y = O, S, Se, Te) with tetrahedral phase and investigate its structural stability, electronic properties, and SHG responses using first-principles calculations. Phonon spectrum analysis confirms that three of these monolayers, HfOSe, HfOTe, and HfSTe, are dynamically stable semiconductors. All three dynamically stable Janus monolayers are indirect semiconductors with valence band maxima located at the Γ point and conduction band minima at the S point. HfOSe and HfSTe monolayers exhibit pronounced out-of-plane SHG with nonlinear susceptibility components reaching the order of 10 2 pm/V, 1–2 orders of magnitude higher than in conventional bulk nonlinear optical crystals, and the charge asymmetry also enhances the SHG response of HfOSe and HfSTe beyond that of their pristine counterparts. By uncovering the intrinsic origins of their outstanding SHG activity, our findings position tetragonal Janus monolayers as a promising materials platform, stimulating the exploration of multiphase Janus systems and their implementation in cutting-edge nonlinear photonic devices.