The main characteristics of the third harmonic emission generated by femtosecond laser pulses propagating in air are investigated by numerically solving the coupled nonlinear Schrodinger equations. Strong third harmonic emission is observed with a maximum conversion efficiency as high as 0.43%. The on-axis phase difference between fundamental beam and third harmonic is investigated. The result is in good agreement with the phase-locking mechanism. Dependence of the conversion of third harmonic emission on focusing conditions is also studied. The results are also compared with those of experiments.
Two interacting light filaments with different initial phases propagating in air are investigated numerically by using a ray tracing method. The evolution of the rays of a filament is governed by a potential field. During propagation, the two potential wells of the two filaments can merge into one or repel each other, depending on the initial phase difference between the two filaments. The study provides a simple description of the interacting filaments.
