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 10¹⁰–10¹⁶ ions/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 1.5×10¹⁵ ions/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.