Polaronic enhancement of second-harmonic generation in lithium niobate

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 when localized at NbV-VLi defect pairs. This is essentially a consequence of the formation of polaronic states inside the band gap. The population of these levels, for example, by illumination leads to strongly enhanced χ⁽²⁾ coefficients and thus allows for the spatial and transient modification of the second-harmonic generation in macroscopic samples.