Generation of highly tunable microwave radiation via a relativistic ionization front

A novel method of continuously upshifting the frequency of an electromagnetic wave is presented. This technique utilizes a relativistically propagating ionization front to simultaneously upshift and compress the source radiation. Using existing technology, it is possible to apply this technique to obtain a single source that is tunable from the microwave (30 GHz) to the millimeter (300 GHz) regime. Peak powers in the kilowatt range with pulse durations of much less than 1 ns are feasible. This technique has been demonstrated in a proof-in-principle experiment by upshifting 34.8-GHz radiation to greater than 116 GHz, with the degree of upshift being continuously tunable. The experimental arrangement employed a microwave cavity filled with azulene vapour. It should be possible to scale the technique to higher frequencies and higher powers.<>