Structures for an optical coupler and methods of fabricating a structure for an optical coupler. A first plurality of segments are positioned with a first spaced arrangement along a longitudinal axis, and a second plurality of segments are positioned with a second spaced arrangement along the longitudinal axis between the first plurality of segments and a waveguide core. A slab layer has a plurality of sections respectively connected to the second plurality of segments. The second plurality of segments have a first thickness, and the slab layer has a second thickness that is less than the first thickness.