Ferroelectric lithium tantalate (${\mathrm{LiTaO}}_3$) is widely used in electro-optic modulators and acousto-optic devices due to its strong nonlinear optical properties. However, unlike other electro-optic materials, the fundamental origin of its electro-optic response remains relatively unexplored. In this work, we use first-principles calculations to investigate the origin of the linear electro-optic effect in ${\mathrm{LiTaO}}_3$ by analyzing its electronic, ionic, and piezoelectric contributions to the electro-optic tensor. We find that the dominant component, $r_{33}$, is not primarily influenced by low-frequency soft phonon modes, as is typical in titanate-based materials. Instead, it is strongly affected by a specific frequency $A_1$ vibrational mode that significantly alters the Ta-O bond. Furthermore, we assess the piezoelectric contribution by calculating the piezoelectric strain tensor and the elasto-optic tensor. This study reveals the fundamental mechanisms driving the electro-optic response in trigonal ${\mathrm{LiTaO}}_3$ and provides insights for designing advanced nonlinear optical materials.

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• Electro-optic effects
• Phonons
• Piezoelectrics
• Density functional calculations