Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb₃SeI₇

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb₃SeI₇, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb₃SeI₇ with C_6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χ_zxx. Meanwhile, although odd-layered Nb₃SeI₇ with C_3v symmetry has both OP and IP dipole components (χ_zxx and χ_yyy), the value of χ_zxx is 1 order of magnitude greater than that of χ_yyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χ_zxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb₃SeI₇ can facilitate the rapid development of chip-integrated NLO devices.