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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.