An improved device, method, and system efficiently couple high-frequency energy from radiation-assisted field emission. A radiation source radiates an emitting surface with an electromagnetic field. The electromagnetic field reduces the potential barrier at the emitting surface, allowing electrons to tunnel from the surface. The tunneling electrons produce a current. The electron tunneling current oscillates in response to the oscillations of the electromagnetic field radiation. Two or more electromagnetic fields of different frequencies radiate the emitting surface, causing photomixing. The electron tunneling current oscillates in response to the difference of the frequencies of the electromagnetic fields.