Relativistic Plasma-Wave Excitation by Collinear Optical Mixing

The relativistic plasma wave excited when the frequency difference between two copropagating C${\mathrm{O}}_2$ laser beams equals the plasma frequency is detected for the first time. The plasma-wave frequency, wave number, spatial extent, and saturation time are directly measured by use of 7-mrad, collective, ruby Thomson scattering and the forward-scattered ir spectrum. The wave amplitude $\frac{\tilde{n}}{n_0}$ is inferred to be (1-3)% which gives a longitudinal electric field of 0.3 to 1 GV/m at a laser intensity of 1.7×${10}^{13}$ W/${\mathrm{cm}}^2$, in reasonable agreement with theory.