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 second-order susceptibility $\chi^{(2)}$ lithium niobate (LiNbO$_3$). Defect trapped polarons and bipolarons are found strongly enhance nonlinear material, in particular if localized at Nb$_\mathrm{V}$-V$_{\mathrm{Li}}$ defect pairs. This essentially consequence polaronic excitation resulting relaxation-induced gap states. The occupation these levels leads enhanced coefficients allows for spatial transient modification second-harmonic generation macroscopic samples.