Linear electro-optic effect in trigonal LiNbO₃: a first-principles study

Lithium niobate LiNbO₃ (LN) has emerged as a promising electro-optic material with applications in silicon photonics. It demonstrates a fairly large linear electro-optic response, often referred to as a Pockels effect. In this talk, we report a first-principles investigation of the origins of the Pockels response in ferroelectric R3c LN. We examine three contributions to the Pockels tensor, its electronic, ionic (Raman), and piezoelectric components. Unlike the BaTiO₃ case, where the Pockels response primarily arises from very low phonon frequencies, the Raman susceptibility terms can be attributed to specific A₁ modes that have a significant impact on the Nb-O bonding length. We compare our Raman susceptibility results obtained from the finite difference method and the density functional perturbation theory. 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 r₃₃ is dominated by the ionic response, which explains a relatively weak frequency dependence of the EO effect in LN.