Amorphous silicon a-Si was made by ion irradiation of crystalline silicon with 1×1015 Xe ions cm–2 at 77 K in the 1–4 MeV energy range. Thermal relaxation of the amorphous network at 500 °C for 1 h leads to an amorphous layer with a refractive index of n = 3.73, significantly higher than that of crystalline silicon (n = 3.45 l at = 1.55 µm). a-Si can thus serve as a waveguide core in Si based optical waveguides. Channel waveguides were made by anisotropic etching of a 1.5 µm silicon-on-insulator structure that was partly amorphized. Transmission measurements of these waveguides as function of the amorphous silicon length show that the a-Si part of the waveguides exhibit a modal propagation loss of 70 cm–1 (0.03 dB µm–1) and a bulk propagation loss of 115 cm–1 (0.05 dB µm–1). Losses due to sidewall roughness are estimated, and are negligible compared to the modal loss.