First-principles calculations are performed to investigate the electro-optic (EO) and elasto-optic effects of the three-dimensional (bulk) and two-dimensional (monolayer) ferroelectric ${\mathrm{NbOI}}_2$. Remarkably large linear EO and elasto-optic coefficients are discovered in both systems, when under stress-free conditions. We further found that the EO responses of bulk and monolayer ${\mathrm{NbOI}}_2$ can be further enhanced with epitaxial strain, because of a strain-driven ferroelectric-to-paraelectric transition that originates from the softening of some polar optical modes. Our findings thus point out that ${\mathrm{NbOI}}_2$, as well as other niobium oxide dihalides, are highly promising for paving the way for potentially efficient nonlinear optical device applications.

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• Electric polarization
• Electro-optic effects
• Nonlinear optics
• Phase transitions
• Ferroelectrics
• Density functional theory
• First-principles calculations