Intrinsic large, flat saturation surfaces of two-dimensional materials are ideal optical and sliding ferroelectric films. Here, we propose the inversion symmetry of a $\mathrm{PtS}{\mathrm{e}}_2$ bilayer can be broken in the ${60}^{\circ }$ twisted structure with an energy difference of 0.05 eV/($\mathrm{PtS}{\mathrm{e}}_2$), compared to the original AA stacking structure. In addition, it exhibits a strong second harmonic generation response as high as 369 pm/V, more than 2.6 times greater than that of hexagonal boron nitride (h-BN) and 1.5 times greater than that of $\mathrm{Mo}{\mathrm{S}}_2$. The out-of-plane component is 162 pm/V, exceeding even that of difficult-to-synthesize Janus MoSSe. Moreover, it also introduces strong ferroelectric polarization of 1.72 pC/m with a low switch barrier of only 32 meV. These characteristics imply it is easier to identify and tune ferroelectrics than widely explored h-BN. In addition, ${\mathrm{PtSe}}_2$ bilayer in the ${60}^{\circ }$ twisted structure also shows a strong bulk photovoltaic effect with a tunable shift current up to $14.9\mu \mathrm{A}\text{/}{\mathrm{V}}^2$. Such a twisting operation can trigger the emergence of physical properties from nothingness and achieves significant regulation. It has excellent potential for both ferroelectric devices and nonlinear optics.

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• Electric polarization
• Ferroelasticity
• Ferroelectricity
• 2-dimensional systems
• Twisted heterostructures
• First-principles calculations
• Optical second-harmonic generation