In one example, a composite processor includes a circuit board, a first processor element package, and a second processor element package. The circuit board has an optical link and an electrical link. The first processor element package includes a substrate with an integrated circuit, a sub-wavelength grating optical coupler, and an electrical coupler coupled to the electrical link of the circuit board. The second processor element package includes a substrate with an integrated circuit, a sub-wavelength grating optical coupler, and an electrical coupler coupled to the electrical link of the circuit board. The sub-wavelength grating optical coupler of the first processor element package, the optical link of the circuit board, and the sub-wavelength grating optical coupler of the second processor element package collectively define an optical communications path between the substrate of the first processor element package and the substrate of the second processor element package.