Simulation of photon acceleration in a plasma wake

The frequency shifting of an ultrashort (femtosecond) low intensity laser p ulse in the presence of a plasma wave is investigated using particle simula tions. One-dimensional simulations confirm the existence of photon trajecto ries similar to the trajectories of trapped or untrapped charged particles in a plasma wave. In the case when a plasma wake is produced by a relativis tically intense laser pulse with a duration of the order of one plasma peri od, some full oscillations of the photon frequency take place only for the untrapped photons moving backward in the plasma wave frame before the inten se laser pulse depletion due to the energy transfer to the plasma wake occu rs. An analytical estimate of the probe pulse phase and frequency shift in two-dimensional (2-D) axially symmetrical plasma wake is performed. Axialll ir symmetrical particle simulations in 2-D, with experimentally attainable parameters, display a frequency shift of the probe pulse of a few percent. In these analytical estimates and 2-D simulations special attention has bee n given to the modeling of the probe pulse optical collecting line that is typically used in plasma diagnostics. (C) 1999 American Institute of Physic s. [S1070-664X(99)03102-X].