Anomalous In-Plane Anisotropic Harmonic Generation in Few-Layer PdSe₂

Anisotropic nonlinear optical effects, including second- and third-harmonic generation (SHG and THG), in two-dimensional (2D)-layered materials are critical for developing on-chip nonlinear photonic and polarization-sensitive devices. Here, we investigate the layer-dependent anisotropic SHG and THG in a few-layer palladium diselenide (PdSe₂). Bilayer PdSe₂ exhibits a 4-fold symmetric polar response, whereas four-layer PdSe₂ displays a distinctive dumbbell-shaped polar pattern under parallel pump-SHG polarization configurations. By tuning the pump polarization direction, we reveal pronounced differences in SHG polarization orientation and ellipticity between the bilayer and four-layer samples. Notably, in four-layer PdSe₂, the SHG polarization direction is strongly coupled with the excitation ellipticity, producing orthogonal SHG orientations under left- and right-circularly polarized excitation. We also observe pronounced layer-dependent THG anisotropy, with the THG amplitude in bilayer PdSe₂ exceeding that of monolayer molybdenum disulfide by more than 60-fold. Harnessing this intrinsic anisotropy, we demonstrate a simple yet effective approach to control SHG polarization directly through pump pulses without additional optical components. This study enhances the fundamental understanding of nonlinear optical anisotropy in 2D materials with implications for the design of ultracompact nonlinear optical devices featuring specific polarization properties.