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A method for up-shifting the frequency of a subpicosecond laser pulse that utilizes the interaction of the pulse with a co-propagating relativistic ionization front is examined. The induced frequency shift is found to initially scale linearly with the propagation time. Asymptotically, the frequency scales as ^1/2. Phase-slippage limitations may be overcome by appropriately increasing the plasma density as a function of, thus allowing for substantially higher-frequency shifts.
Esarey et al. (1991) studied this question.