Nonlinear interaction of femtosecond laser pulses with a CO₂-laser-induced air spark

We observe a dramatic enhancement in third harmonic generation (THG) when intense femtosecond laser pulses travel through CO₂-laser-induced spark in air. By further investigating the dependence of the enhancement factor on the delay between the femtosecond laser pulse and CO₂ laser pulse in various gases at different pressures, we reveal that the enhancement in THG results from the suppression of Gouy-phase-induced destructive interference, which can be attributed to a fast and a slow process. Our analyses show that the plasma generated in CO₂ laser field plays the dominant role in the fast process whereas hydrodynamics of the gas heated by the CO₂ laser contributes to the slow process.