Fourth-order harmonic generation during parametric four-wave mixing in the filaments in ambient air

We propose a simplified scheme for efficient 200-nm pulse generation in isotropic medium using 800-nm radiation. The observation and optimization of the fourth-order harmonic generation during two-color filamentation in air are reported. This parametric process is based on four-wave mixing of the fundamental and the second-order harmonic radiation of the Ti:sapphire laser. A systematic study of the influence of the laser intensity, the polarization, the chirp, and the pulse duration on the fourth-order harmonic output from the filaments has been carried out. The fourth-order harmonic conversion efficiency was estimated to be of the order of 10^-4.