Frequency upconversion of electromagnetic radiation upon transmission into an ionization front

We have demonstrated that an underdense ionization front can significantly upshift the frequency of an impinging electromagnetic wave. Source radiation at 35 GHz was shifted to more than 116 GHz when a relativistically propagating, laser-produced ionization front interacted with microwave radiation that was confined in a resonant cavity. The amount of upshift was proportional to the plasma density in the front, in agreement with theoretical predictions.