Impact of ion exchange on vibrational modes in Rb-doped KTiOPO4: A Raman spectroscopy study on the interplay between ion exchange and polarization switching

Recently, ion exchange (IE) has been used to periodically modify the coercive field (Ec) of the crystal prior to periodic poling, to fabricate fine-pitch domain structures in Rb-doped KTiOPO4 (RKTP). Here, we use micro-Raman spectroscopy to understand the impact of IE on the vibrational modes related to the Rb/K lattice sites, TiO octahedra, and PO4 tetrahedra, which all form the basis of the RKTP crystal structure. We analyze the Raman spectra of three different RKTP samples: (1) a RKTP sample that shows a poled domain grating only, (2) a RKTP sample that has an Ec grating only, and (3) a RKTP sample that has both an Ec and a domain grating of the nominally same spacing. This allows us to determine the impact of IE on the vibrational modes of RKTP. We characterize the changes in the lower Raman peaks related to the alkali-metal ions, as well as observe lattice modifications induced by the incorporation of Rb+ that extend further into the crystal bulk than the expected IE depth. Moreover, the influence of IE on the domain walls is also manifested in their Raman peak shift. We discuss our results in terms of the deformation of the PO4and TiO groups. Our results highlight the intricate impact of IE on the crystal structure and how it facilitates periodic poling, paving the way for further development of the Ec-engineering technique.