Third-harmonic generation from regularized converging filaments

Third-harmonic generation was investigated along four regularized filaments obtained by sending a 744 nm ultrashort laser pulse through an amplitude mask with four holes. Experiments show that the third-harmonic angular distribution forms a square-like pattern, which is reproduced by three-dimensional nonstationary numerical simulations. The fusion of the initially separated filaments and the formation of a superfilament arrests the third-harmonic yield but enhances self- and cross-phase modulation effects and produces a fourfold increase in the high-frequency wings of the spectra of the fundamental and the third harmonic. This demonstrates the opposite effect of superfilamentation on nonlinear phenomena that require a sufficiently large coherence length or the intense localized response.