Abstract

As an ionizing laser pulse passes through a neutral medium, it produces a boundary between the neutral and ionized regions that propagates at the group velocity of the laser pulse in the ionized medium. Developments in laser technology have allowed the production of such a boundary, or ionization front, via photo-ionization of neutral gas. If electromagnetic radiation impinges upon such a front and is reflected, its frequency will be upshifted by the relativistic Doppler effect [1]. For highly relativistic ionization fronts, the frequency could be upshifted by many orders of magnitude. Unfortunately, the density of the ionized gas, or plasma, required to reflect the incident radiation is extremely large for highly relativistic fronts. However, it has recently been proposed that even for modest plasma densities, where the radiation passes through the front rather than reflecting from it, large frequency upshifts are possible [2]. In this case, the degree of upshift, which is proportional to the plasma density in the front, can be continuously tuned by varying the neutral gas density. Also, because the spatial extent of the front can be easily controlled, production of sub-femtosecond pulses using this technique has been proposed [2].

© 1992 The Author(s)