Polaronic Enhancement of Second-Harmonic Generation in Lithium Niobate

TL;DR

This study uses density-functional theory to show that defect-trapped polarons and bipolarons significantly enhance the second-harmonic generation in lithium niobate by creating relaxation-induced gap states that boost nonlinear susceptibility.

Contribution

It reveals how defect-induced polarons and bipolarons can be used to enhance nonlinear optical properties in lithium niobate, a novel insight into defect engineering for photonics.

Findings

Polarons and bipolarons strongly increase $oldsymbol{ ext{χ}}^{(2)}$ in lithium niobate.

Localized defect pairs create gap states that enhance nonlinear susceptibility.

Spatial and transient control of second-harmonic generation is possible through defect manipulation.

Abstract

Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility $\chi^{(2)}$ of lithium niobate (LiNbO$_3$). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at Nb$_\mathrm{V}$-V$_{\mathrm{Li}}$ 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 $\chi^{(2)}$ coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.