Evidence of Ionization Blue Shift Seeding of Forward Raman Scattering

Abstract

We report on the results of spectroscopic experiments that were conducted by focusing an intense ultra‐short laser pulse onto a helium gas target. The scattered light from the interaction region was measured spectrally and spatially from various directions as a function of laser intensity and plasma density. The experimental data showed that forward Stimulated Raman Scattering (SRS) was sensitive to the focus position of laser relative to the nozzle. Together with the plasma channel that was imaged by a CCD camera, the measurements indicate that SRS is seeded by the ionization blue‐shifted light. The cross‐phase modulation between the SRS and laser beam was also observed in the experiment. © 2004 American Institute of Physics