Disclosed are structures and methods directed to waveguide structures exhibiting improved device performance including improved attenuation of scattered light and/or transverse magnetic modes. In an illustrative embodiment according to the present disclosure, a rib waveguide structure including a rib overlying a slab waveguide (or superimposed thereon) is constructed wherein the slab waveguide is heavily doped at a distance from the rib which has a very low overlap with rib guided modes. Advantageously, such doping may be of the P-type or of the N-type, and dopants could be any of a number of known ones including—but not limited to—boron, phosphorous, etc.—or others that increase optical propagation loss. As may be appreciated, the doped regions advantageously absorb scattered light which substantially improves the structures' performance.