Extra ‘‘strange’’ modes in ion implanted lithium niobate waveguides

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Abstract
An extra ‘‘strange’’ mode has been observed in the extraordinary index mode spectrum of certain optical waveguides produced by He+ ion implantation in LiNbO3. This behaves differently to the normal modes contained within the main optical well of the waveguide, for example when observed at varying wavelengths or after surface polishing. The mode has been shown to be a real guided‐wave mode, and has been attributed to a subsidiary optical well located beyond the main nuclear damage optical barrier. It is suggested that this well is produced by the radiation‐enhanced diffusion of Li+ towards the nuclear damage barrier, and the mechanism is compared with that responsible for the well on the near side of the barrier which causes the previously reported ‘‘missing’’ modes.