In one embodiment, the apparatus includes a substrate having a surface and an optical waveguide having a ridge-shaped semiconductor optical core, the ridge-shaped semiconductor optical core being located over the surface. The apparatus may further include a first semiconductor slab being in contact with a first portion of the ridge-shaped semiconductor optical core, and a second semiconductor slab being in contact with a second portion of the ridge-shaped semiconductor optical core, the second semiconductor slab being farther from the surface than the first semiconductor slab.