Specific behavior of refractive indices in low-dose He+-implanted LiNbO3 waveguides

Y - or Z- cut LiNbO3 crystals were implanted at room temperature by helium with energy in the MeV range and various doses with a specific care for the 1010–1016ions∕cm2 range. The induced structure defects were investigated by micro-Raman and IR reflectivity. IR Brewster angle technique was applied for detailed measurements on both indices variations in nuclear collision damaged areas. It is established that a transition from point to extended defects occurs at some threshold dose around 1.5×1015ions∕cm2 and that the extraordinary index is increased in a specific dose range around this threshold. A model based on depolarized cluster formation is proposed to explain the difference observed between the changes of ordinary and extraordinary refractive indices.