Study of anomalous behaviour of LiTaO3 during the annealed proton exchange process of optical waveguide’s formation – comparison with LiNbO3

This paper deals with a detailed study of changes that lithium tantalate (LT) and lithium niobate (LN) single crystals undergo during the annealed proton exchange (APE) process of optical waveguides’ formation. It is a well-known fact that several cases of anomalous behaviour are connected to the APE:LT samples, bringing thus an obstruction for the practical utilization of the APE:LT waveguides. As the LT crystal possesses even better optical properties than the LN crystal (e.g., it is less susceptible to optical damage), it is desirable to provide research focused on its behaviour during the APE process in order to acquire a control over the fabrication of the APE:LT devices. Neutron depth profiling (NDP), elastic recoil detection analysis (ERDA) and heavy ion ERDA (HI-ERDA) were performed to study changes in the surface of the LT and LN Z-cut wafers caused by the APE treatment and to determine the concentration depth profiles of the exchanged ions (lithium and hydrogen). Information on modifications of the crystals during the APE was obtained using X-ray diffraction (XRD) analysis. Optical/waveguiding properties of the samples were obtained by means of the standard mode spectroscopy at 633 nm. The experiments proved that the LT is significantly less affected by the APE process compared to the LN and that most characteristics of the APE:LT layers can be easily restored towards that of the virgin crystal by the annealing process.