In depth Raman analysis of the ferroelectrics KTiOPO4 and LiNbO3: role of domain boundaries and defect

In the context of ferroelectrics spatially resolved Raman spectroscopy is a powerful tool to investigate stoichiometry, defects or the ferroelectric properties, as well as to visualize domain structures or waveguides. Using Raman spectroscopy for investigations requires a throughout understanding of the spectra and underlying mechanisms. For example, in the context of the common nonlinear materials, lithium niobate and potassium titanyl phosphate, no comprehensive understanding of the Raman spectra of the bulk materials is available, while the underlying mechanism of the domain wall contrast in Raman spectroscopy is not well understood. In this work, questions like these have been addressed in terms of systematic experimental investigations in close cooperation with density functional theory. In particular, it was possible to present a complete assignment of all phonons in the lithium niobate system, which serves as the basis for the understanding of the domain wall spectrum. Here, the domain wall spectrum can be explained with regard to microscopic structural effects, such as strains and electric fields, as well as a macroscopic change of selections rules. Both mechanisms are likewise present in the domain wall spectrum, while being present at different length scales. In the context of potassium titanyl phosphate the first throughout Raman investigations of domain structure, waveguides and periodically poled waveguides are presented. In the context of Rb-exchanged waveguides the change in stoichiometry, but also effects of strain are detected. Here, the Raman analysis provides a method to evaluate these effects.