Second-order nonlinearities in the domain walls of periodically poled KTiOPO4

The domain wall regions in periodically poled KTiOPO4 crystals were examined and found to give rise to phasematched second harmonic generation in the Čerenkov directions. This phenomenon is caused by the nonlinear coefficients d11 and d12, which are not present in single domain regions, but are nonzero at and close to domain walls. The appearance of these nonlinearities is attributed to strain, produced by the domain inversion process and results in the creation of a dc piezoelectric field.