Resonant Self-Focusing of Laser Light in a Plasma

When laser light containing two frequencies ${\omega }_1$ and ${\omega }_2$ impinges on a plasma with ${\omega }_p={\omega }_1-{\omega }_2$ plasma waves are resonantly excited. The ponderomotive force of these waves is much larger than that of the beam, and the self-focusing effect is greatly enhanced. With use of C${\mathrm{O}}_2$ laser light of only 5 × ${10}^9$ W/${\mathrm{cm}}^2$ intensity and a plasma at 0.6% of critical density $n_c$, this effect is observed as a strong (${10}^5$-fold) enhancement of refracted light at $\theta =5\mathrm{}°-12\mathrm{}°$ when the density is at the resonant value.