According to one embodiment, a surface-plasmon (SP) beam generated by an SP source and directed via an SP waveguide is applied to a gate node of a field-effect transistor (FET). The FET also has a source node and a drain node. In a representative configuration, the gate, source, and drain nodes are electrically biased to pass an electrical current between the source and drain nodes in a manner that makes the electrical current responsive to the intensity of the SP beam.