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 ${10}^{-4}$.

Figure 1

(a) Spectral shapes of the fundamental radiation after propagation through the focal area at low (thin curve) and high (thick curve) energies of laser pulses. (b) Spectral distribution of the fourth-order harmonic.

Figure 2

(a) Fourth-order harmonic output as a function of pulse duration for positively and negatively chirped laser radiation. Positive and negative values of pulse duration correspond to positively and negatively chirped pulses. (b) Harmonic spectra at optimal conditions for the FHG.

Figure 3

Dependences of the fourth-order harmonic output on the (a) fundamental and (b) second-order harmonic intensity.