We report the fabrication of periodically poled domain patterns in x-cut lithium niobate thin-film. Here, thin films on insulator have drawn particular attention due to their intrinsic waveguiding properties offering high mode confinement and smaller devices compared to in-diffused waveguides in bulk material. In contrast to z-cut thin film lithium niobate, the x-cut geometry does not require back electrodes for poling. Further, the x-cut geometry grants direct access to the largest nonlinear and electro-optical tensor element, which overall promises smaller devices. The domain inversion was realized via electric field poling utilizing deposited aluminum top electrodes on a stack of LN thin film/SiO2 layer/Bulk LN, which were patterned by optical lithography. The periodic domain inversion was verified by non-invasive confocal second harmonic microscopy. Our results show domain patterns in accordance to the electrode mask layout. The second harmonic signatures can be interpreted in terms of spatially, overlapping domain filaments which start their growth on the +z side.

Confocal Raman spectroscopy is applied to identify ferroelectric domain structure sensitive phonon modes in potassium titanyl phosphate. Therefore, polarization-dependent measurements in various scattering configurations have been performed to characterize the fundamental Raman spectra of the material. The obtained spectra are discussed qualitatively based on an internal mode assignment. In the main part of this work, we have characterized z-cut periodically poled potassium titanyl phosphate in terms of polarity-and structure-sensitive phonon modes. Here, we find vibrations whose intensities are linked to the ferroelectric domain walls. We interpret this in terms of changes in the polarizability originating from strain induced by domain boundaries and the inner field distribution. Hence, a direct and 3D visualization of ferroelectric domain structures becomes possible in potassium titanyl phosphate. V

In recent years, Raman spectroscopy has been used to visualize and analyze ferroelectric domain structures. The technique makes use of the fact that the intensity or frequency of certain phonons is strongly influenced by the presence of domain walls. Although the method is used frequently, the underlying mechanism responsible for the changes in the spectra is not fully understood. This inhibits deeper analysis of domain structures based on this method. Two different models have been proposed. However, neither model completely explains all observations. In this work, we have systematically investigated domain walls in different scattering geometries with Raman spectroscopy in the common ferroelectric materials used in integrated optics, i.e., KTiOPO 4 , LiNbO 3 , and LiTaO 3. Based on the two models, we can demonstrate that the observed contrast for domain walls is in fact based on two different effects. We can identify on the one hand microscopic changes at the domain wall, e.g., strain and electric fields, and on the other hand a macroscopic change of selection rules at the domain wall. While the macroscopic relaxation of selection rules can be explained by the directional dispersion of the phonons in agreement with previous propositions, the microscopic changes can be explained qualitatively in terms of a simplified atomistic model.

Second-harmonic (SH) microscopy is a widely used tool for the study of ferroelectric domains, domain walls, and their substructure. Yet, the contrast mechanism, particularly for the commonly used large numerical aperture, is not fully understood. In this work, we examine the contrast mechanism of SH microscopy in periodically poled LiNbO3 for the case of tightly focused laser beams and in the surface-near regime. The results are interpreted along theoretical calculations that include a vectorial field model for excitation and detection. Our model suggests that the characteristic contrasts mainly originate from interference patterns in the signal due to the sign change of the nonlinear susceptibility at the domain boundary. We find that for large numerical apertures, the tight focusing induces polarization components (axial and orthogonal to incident polarization), and the subsequent mixing of differently polarized light fields via off diagonal tensor elements plays an important role for the domain wall contrast. With our model-based analysis, this work represents the foundation for the investigation of the substructure of domain walls with second-harmonic microscopy.

In this work, the second-harmonic (SH) signal generated by nonlinear optical crystals is studied in the tightly focused regime. The experimental approach is based on an adapted focal imaging technique, which allows the mapping of the SH intensity distribution in the back focal plane via a traversable pinhole in the confocal operation mode. On the theoretical side, a vectorial treatment of the involved optical fields enables the description and interpretation of the occurring interactions by taking into account the applied experimental parameters. The theoretical results are exemplarily validated by comparison to the acquired experimental data gained by the examination of LiNbO3 and KTiOPO4. It is shown how the phase and amplitude of vector components of the incoming electromagnetic field in the focus as well as the local optical properties of the nonlinear optical crystals determine the characteristic nonlinear signals in the back focal plane.