Structures for a wavelength-division multiplexing filter and methods of fabricating a structure for a wavelength-division multiplexing filter. The structure includes a first waveguide core, a second waveguide core laterally spaced from the first waveguide core, and a ring resonator arranged in a vertical direction over the first waveguide core and the second waveguide core. The ring resonator is also arranged in a lateral direction between the first waveguide core and the second waveguide core. The first and second waveguide cores are composed of a semiconductor material, such as single-crystal silicon, and the ring resonator is composed of a dielectric material, such as silicon nitride.