Frequency up-shifting of laser pulses by copropagating ionization fronts

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 τ. Asymptotically, the frequency scales as ${\mathrm{\tau }}^{1/2}$. Phase-slippage limitations may be overcome by appropriately increasing the plasma density as a function of τ, thus allowing for substantially higher-frequency shifts.