Tailoring of optical planar waveguides in KNbO₃ by MeV He ion implantation

Abstract

In KNbO₃ single crystals planar optical waveguides were fabricated by He ion implantation with doses from 2·5 · 10¹⁴ cm⁻² to 2 · 10¹⁶ cm⁻². The waveguides were analyzed by dark line spectroscopy at 633 nm and the detailed refractive index profiles were reconstructed with a dark mode reflectivity calculation method. We found a decrease of all three refractive indices n_a, n_b and n_c in a layer several μm below the crystal surface. At a dose of 10¹⁶ cm⁻² saturation of the index decrease of Δn_a = − 0·160, Δn_b = − 0·212 and Δn_c = − 0·048 occur indicating the formation of an optically isotropic layer with a refractive index of 2·119. In the surface close region of the waveguide an anomalous increase of the smallest index n_c of Δn_c = + 0·013 was observed. The indices of KNbO₃ are ten times more sensitive to radiation damage in comparison with LiNbO₃.