Second-harmonic imaging of ferroelectric domain walls

Domain walls in periodically poled ferroelectric ${\mathrm{KTiOPO}}_{\mathrm{4}}$ and ${\mathrm{LiNbO}}_{\mathrm{3}}$ crystals are observed by making use of second-harmonic (SH) generation enhancement in the transition regions between neighboring domains. SH images of domain walls obtained with various samples for different polarization configurations are presented. The SH generation enhancement is found especially pronounced for the polarization of the SH radiation being perpendicular to the domain walls. The origin and selection rules for the contrast in SH images of domain walls are discussed. The results obtained suggest that the domain walls produce a deteriorating effect on SH generation by quasiphase matching.