Structures for a waveguide bend and methods of fabricating a structure for a waveguide bend. A waveguide bend is connected to a waveguide core. A slab layer, which is thinner than the waveguide bend, is coupled to the waveguide core and the waveguide bend. The slab layer includes a first curved opening and a second curved opening that is positioned between the first curved opening and a side surface of the waveguide bend. A section of the slab layer is positioned between the first and second curved openings. The first curved opening has a first radius, and the second curved opening has a second radius that is greater than or less than the first radius of the first curved opening.