Three-dimensional observations of polar domain structures using a confocal second-harmonic generation interference microscope

The necessary conditions for nondestructive and three-dimensional (3D) observations of inverted polar domain structures using the second-harmonic generation interference microscope (SHGIM) are examined experimentally and theoretically using MgO doped congruent LiNbO3 and periodically poled (PP) MgO doped near stoichiometrical LiNbO3. The results can be explained well by the Boyd and Kleinman theory of strongly focused optical systems. To obtain clear 3D images of domain structures by SHGIM, the sign of the wave number Δk offset of the fundamental and SH waves should be negative and the SHG tensor components d31 should be used for both samples when they are observed with a wavelength of 1064 nm at room temperature. Under these conditions, 3D PP domain structures are clearly revealed by SHGIM. General conditions for the 3D visualization of polar domain structures are discussed for the cases of sum-frequency and difference-frequency generations.