Structures for a photodetector and methods of fabricating a structure for a photodetector. The structure includes a first waveguide core having a first taper, a semiconductor layer having a sidewall adjacent to the first taper, and a second waveguide core having a second taper that is positioned to overlap with the first taper and a curved section. The second taper is longitudinally positioned between the sidewall of the semiconductor layer and the curved section. The curved section terminates the second waveguide core.