= Periodically poled lithium niobate
<Periodically poled lithium niobate> is <lithium niobate> whose ferroelectric domain orientation is reversed periodically along an optical interaction path. Reversal changes the sign of the relevant second-order <nonlinear optical susceptibility>, supplying the spatial modulation needed for <quasi-phase-matching>.
For a uniform collinear conversion mismatch $\Delta k$, a first-order poling period $\Lambda$ can compensate it through $\Delta k-2\pi/\Lambda\simeq0$, with signs chosen consistently. The generated contributions then reinforce over many domains instead of repeatedly canceling. The poling period, <temperature> and input <wavelengths> determine the useful conversion band; domain defects and finite length affect efficiency. https://covesion.com/knowledge-hub/technical-guide-material-properties-of-lithium-niobate/[Covesion manufacturer technical guide: nonlinear coefficients and periodically poled lithium niobate]{ref}
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