Frequency Upshifting of Electromagnetic Radiation via AN Underdense Relativistic Ionization Front

An underdense, relativistically propagating ionization front has been utilized to upshift the frequency of an impinging electromagnetic wave from 35 GHz to more than 173 GHz in a continuously tunable fashion. The source radiation interacted with the ionization front inside a metallic waveguide. The front, simply a moving boundary between ionized and neutral gas, was created as a short, intense pulse of ionizing laser radiation propagated through the gas-filled waveguide. In 1991, W. B. Mori showed theoretically that large upshifts are possible using underdense ionization fronts (underdense implies that the plasma density is lower than that required to reflect the source radiation), where the source wave is transmitted through the plasma/neutral boundary. We have extrapolated Mori's analysis to interactions within a waveguide. We launched source radiation both along and opposite to the direction of propagation of the front. Because the group velocity of the source radiation inside the waveguide was only half that of the ionization front, both the counter- and co-propagating source radiation were overtaken and transmitted into the front. In agreement with the theoretical predictions, the co-propagating wave was upshifted to a higher frequency than the counter-propagating wave for a given plasma density, and both upshifts were proportional to the front's density. The theory also predicts temporal compression of the co-propagating wave as its group velocity increases with frequency. Conversely, the group velocity of the counter-propagating wave is expected to decrease to zero, then increase in the same direction as the front for higher densities, also being significantly compressed for large upshifts. We have experimentally observed the compression of the co-propagating wave to less than 500 psec duration, in good agreement with theory. The duration of the upshifted counter-propagating wave initially increased with plasma density to more than 7 nsec and was less than 2 nsec at higher densities. However, the "reflection" of the counter -propagating wave was not conclusively resolved. This is a new technique for generating high-power, short-pulse, tunable radiation, and has potential applications in areas such as time-resolved microwave spectroscopy, plasma diagnostics, and remote sensing.