Lithium niobate $\mathrm{LiNb}{\mathrm{O}}_3$ (LN) has emerged as a promising electro-optic (EO) material with applications in silicon photonics. It demonstrates a large linear EO response, often referred to as the Pockels effect. We report on a first-principles investigation of the origins of the Pockels response in $R3c$ LN. We examine three contributions to the Pockels tensor, its electronic, ionic (Raman), and piezoelectric components, and demonstrate that the large value of Raman susceptibility can be traced to specific $A_1$ modes that affect the Nb-O bonding. To examine the origin of the piezoelectric Pockels response, we compute the elasto-optic and piezoelectric tensors. Importantly, the largest component of the Pockels tensor $r_{33}$ is dominated by the ionic response, which explains the insensitivity of the EO effect in LN to frequency.

• Received 15 November 2023
• Revised 22 January 2024
• Accepted 5 February 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.025202