A photovoltaic photodetector includes a substrate, a graphene layer, and a dielectric layer positioned between the substrate and the graphene layer. One or more first antenna electrodes includes a first metal in direct contact with the graphene layer. One or more second antenna electrodes includes a second metal in direct contact with the graphene layer. The first and second metals have different work functions. A drain electrode is electrically coupled to the one or more first antenna electrodes, and a source electrode is electrically coupled to the one or more second antenna electrodes. The photovoltaic photodetector can be configured to be operable over a wavelength region of 2 μm to 24 μm and has a response time of 10 ns or less.