Third Harmonic Generation from Perturbed Femtosecond Filaments in Air

We investigate third harmonic generation from femtosecond laser filaments. A two-order-of-magnitude enhancement of third harmonic light is observed when the (signal) filament is perturbed by a second laser (pump) filament. This enhancement is studied as function of laser polarization, time delay between the pump and signal filaments, the pump filament energy, etc. Based on these systematic results, we attribute the enhancement to a quenching of the interference effects of the third harmonic generated in the first and second half of the signal filament. Numerical simulations based on a two-color propagation model reproduce well the experimental observations and confirms our explanation.