Scanning-nonlinear-dielectric-microscopy study on periodically poled LiNbO₃ for a high-performance quasi-phase matching device

Studies of the domain distribution of periodically poled LiNbO₃ (PPLN) using scanning nonlinear dielectric microscopy (SNDM) were performed for high-quality quasi-phase matching (QPM) devices and some very important results were obtained. First, a single domain surface layer formed on PPLN was found and second, the SNDM studies also revealed that a very strong residual stress or an internal electric field remained in PPLN fabricated using the application of a high voltage due to the pinning effect in single crystal LiNbO₃. This stress or internal field substantially reduced the nonlinear dielectric constant of LiNbO₃. By annealing to release this stress, it was confirmed that the LiNbO₃ recovered from the reduced nonlinear dielectric constant. In the optical region, this constant is equivalent to the second harmonic generation constant, therefore, it is proposed that annealing is an effective method for obtaining a highly efficient high-performance QPM device.