Far- and near-field second-harmonic imaging of ferroelectric domain walls

Domain walls in periodically poled ferroelectric LiNbO3 crystals are observed with both far- and near-field imaging techniques that make use of second harmonic generation in the transition regions between neighbouring domains. Second harmonic images of domain walls represent bright lines of about 0.5 micrometers in width (as measured with the near-field microscope) for the polarization of the second harmonic radiation perpendicular to the domain walls.

Origin and selection rules for the constrast in second harmonic images of domain walls are discussed.