As emerging ferroelectric (FE) materials, ultrathin two-dimensional (2D) sliding FEs without a phase-matching bottleneck usually exhibit pronounced second-harmonic generation (SHG) responses. Despite that the structural polarity of sliding FEs can be precisely detected via SHG characterizations, distinguishing the orientations of sliding ferroelectricity based on SHG responses has rarely been realized, as SHG intensities for upward and downward polarization states are supposed to be same. In the current work, combining computational simulations and experimental characterizations, the orientation of sliding ferroelectricity is demonstrated to be readily distinguishable via SHG responses in 2D ${\mathrm{SnP}}_2{\mathrm{S}}_6$ (${\mathrm{SnP}}_2{\mathrm{Se}}_6$), a potential sliding FE material. Specifically, owing to the unique symmetry operation within FE-${\mathrm{SnP}}_2{\mathrm{S}}_6$ (${\mathrm{SnP}}_2{\mathrm{Se}}_6$), the intersection between ${\chi }_{xxx}$ and ${\chi }_{yyy}$ SHG susceptibility coefficients with opposite signs leads to the effective rotation of SHG polar directions upon switching of sliding ferroelectricity. Moreover, the remarkable dependence of SHG polar directions on the orientation of sliding ferroelectricity is further validated by experimental characterizations performed on ${\mathrm{SnP}}_2{\mathrm{S}}_6$ crystal in a single FE domain structural form. In this work, we open an avenue for in situ detecting of the ferroelectricity orientation of 2D sliding FEs based on SHG nonlinear optical responses and demonstrate the controllable optical nonlinearity for slidetronics applications.

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• Ferroelectricity
• Ferroelectrics
• Layered crystals
• Density functional theory
• High-harmonic generation
• Wannier function methods