Third-Harmonic Generation and Self-Channeling in Air Using High-Power Femtosecond Laser Pulses

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.

Figure 1

Numerical results for the on-axis peak intensity of (a) the fundamental pulse and (b) the TH pulse for $P_0=0.1P_{\mathrm{c}\mathrm{r}}$ (solid line) [right-hand scale], $P_0=1.5P_{\mathrm{c}\mathrm{r}}$ (dotted line) and $P_0=3.0P_{\mathrm{c}\mathrm{r}}$ (dashed line) [left-hand scale] as a function of the propagation distance. In (c) the total conversion efficiency as a function of propagation distance is shown. The beam was focused with a lens of $f=100\mathrm{c}\mathrm{m}$.

Figure 2

Experimentally observed pump pulse (circles, [left-hand scale]) and TH pulse (triangles, [right-hand scale]) filament energy, defined as the total energy contained in a 400 μm diameter, as a function of the propagation distance.

Figure 3

Numerical results for the spatiotemporal intensity distribution of the fundamental pulse (left) and TH pulse (right) at (a) $z=97\mathrm{c}\mathrm{m}$, (b) $z=98\mathrm{c}\mathrm{m}$, and (c) $z=101\mathrm{c}\mathrm{m}$. The input power is $P_0=3.0P_{\mathrm{c}\mathrm{r}}$.