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 res...

Nonlinear optics, and particularly second harmonic generation (SHG), is increasingly used in many modern disciplines from material characterization physical sciences to bioimaging medicine optical signal processing information technology. We present a theoretical analysis yielding strong estimate of the energy-integrated SHG response. Compact spherical multipoles are provided for corresponding ...