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

• Received 7 December 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.3570