Tailoring of refractive index profiles in LiNbO3 optical waveguides by low-fluence swift-ion irradiation

Proton-exchange LiNbO3 planar optical waveguides have been irradiated with swift ions (Cl 30 MeV) at very low fluences in the range 5 x 1010-5 x 1012 cm-2. Large modifications in the refractive index profiles, and therefore in the optical performance, have been obtained due to the generation of amorphous nano-tracks by the individual ion impacts. Moreover, a fine tuning of the refractive index can be achieved by a suitable control of the fluence (δn/δφ ∼ 10-14 cm2 or δn ∼ 10-5 for δφ = 109 cm-2). An effective medium approach has been used to account for those changes and determine the amorphous fraction of material. The results have been compared with information extracted from complementary RBS channelling experiments. From the calculated amorphous fractions a radius of ∼2 nm for the amorphous tracks have been estimated