Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility χ(2) of lithium niobate (LiNbO3). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at NbV-VLi defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced χ(2) coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.
@article{arxiv.2106.01145,
title = {Polaronic Enhancement of Second-Harmonic Generation in Lithium Niobate},
author = {Agnieszka L. Kozub and Arno Schindlmayr and Uwe Gerstmann and Wolf Gero Schmidt},
journal= {arXiv preprint arXiv:2106.01145},
year = {2021}
}