A proposed fast-wave photo-electric laser mixer for millimetre-wave generation

A photo-electric cell having the form of a parallel-strip transmission line in which the lower strip is photo-emissive is proposed as a means of generating millimetre waves from two laser beams. The non-emissive strip is maintained at a positive potential with respect to the emissive strip, so that photo electrons are attracted to it. By making the potential as large as possible and the separation of the strips as small as possible the electron transit time is reduced. Two laser beams, differing in frequency by the required millimetric frequency, are directed on to the photo-emissive surface along the strip at almost grazing incidence. Electron emission is proportional to the instantaneous intensity of the two beams together; as this is proportional to the square of the sum of their amplitudes a difference-frequency term (in the millimetric range) appears in the photo-current emitted. As the laser beams travel almost parallel to the strips, the phase velocity of the beams measured along the strips is almost equal to that of a millimetric TEM wave travelling between them. Through the electron-emission process, such a TEM wave is in fact generated, and grows linearly in amplitude with distance along the strips. The theory of the device is worked out, and a numerical example is given.