Abstract: The present disclosure relates to semiconductor structures and, more particularly, to optical via connections in chip-to-chip transmission in a 3D chip stack structure using an optical via, and methods of manufacture. The structure has a first wafer, including a first waveguide coupled to an optical resonator in the first wafer, and a second wafer, including a second waveguide, located over the first wafer. The structure also includes an optical via extending between the optical resonator of the first wafer and the second waveguide of the second wafer to optically couple the first and second waveguides.