Determining the Dipole Orientation of Second Harmonic Generation in 3R-MoS₂ for Enhanced Nonlinear Susceptibility

As an efficient two-dimensional nonlinear optical crystal, 3R-MoS₂ exhibits intrinsic bulk second-order nonlinearity with substantial second harmonic generation (SHG) due to the interfacial charge transfer induced interlayer dipole and intralayer intrinsic asymmetric dipole. However, how these dipoles determine the SHG emission dipole orientation and intensity in 3R-MoS₂ has not been clearly resolved. Here, we accurately determine the coexistence of in-plane and out-of-plane SHG emission dipoles in few-layer 3R-MoS₂ through radial-/azimuthal-polarization excitation SHG measurements and back focal plane (BFP) imaging combined with numerical simulations, where the SHG emission dipole orientation (Θ) in real space for 3L, 4L, 5L, and 6L 3R-MoS₂ is determined to be ∼8°, ∼16°, ∼20°, and ∼32°, respectively. The layer-dependent Θ arises from the significant decrease in SHG susceptibility χ₁₆⁽²⁾ with an increasing layer number, which weakens the contribution of the in-plane SHG emission dipole component to the overall SHG response. Moreover, by tailoring the interlayer charge transfer induced dipole via hydrostatic pressure, more than 1 order of magnitude enhancement of SHG susceptibility in few-layer 3R-MoS₂ has been achieved, which originates from the strengthened interlayer charge transfer and interfacial charge rearrangement upon compression. Our findings not only optimize phase-matching conditions via aligning the emission dipoles but also provide a strategy for fine-tuning the nonlinear optical responses in nanophotonic applications.