Abstract
Unlike other crystals, the counter intuitive response of bismuth germanate crystals ([Formula: see text], BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of [Formula: see text]. The currently accepted view that index changes [Formula: see text] could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to [Formula: see text] of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.