Laser Wakefield Excitation and Measurement by Femtosecond Longitudinal Interferometry

Plasma density oscillations (Langmuir waves) in the wake of an intense ( ${I}_{\mathrm{peak}}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{17}\mathrm{W}/{\mathrm{cm}}^{2}$) laser pulse (100 fs) are measured with ultrafast time resolution using a longitudinal interferometric technique. Phase shifts consistent with large amplitude ( $\ensuremath{\delta}{n}_{e}/{n}_{e}\ensuremath{\sim}1$) density waves at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of $\ensuremath{\sim}10\mathrm{GV}/\mathrm{m}$. Strong radial ponderomotive forces enhance the density oscillations.