Structures including an electro-optical phase shifter and methods of fabricating a structure including an electro-optical phase shifter. The structure includes a waveguide core on a semiconductor substrate, and an interconnect structure over the waveguide core and the semiconductor substrate. The waveguide core includes a phase shifter, and the interconnect structure includes a slotted shield and a transmission line coupled to the phase shifter. The slotted shield includes segments that are separated by slots. The slotted shield is positioned between the transmission line and the substrate.