Fifth-order intensity autocorrelations based on six-wave mixing of femtosecond laser pulses

It is shown both experimentally and by numerical simulations that fifth-order intensity autocorrelations of femtosecond laser pulses can be obtained from two-beam noncollinear six-wave mixing in air. A numerical analysis of competing direct and six-wave-assisted third-harmonic-generation pathways showed that these measurements are suitable for the background-free temporal characterization of laser pulses. Reshaping of the pulse and 10 fs subpulse formation during the primary stages of light filamentation were observed using the proposed method.

Figure 1

Plots of a bimodal test function (solid blue lines) and corresponding second- and fifth-order intensity autocorrelation functions (red dashed lines) on the left and right panels, respectively.

Figure 2

Left: Configuration of the pump $I_L\left(t\right)$ and THG $I_{\mathrm{THG}}\left(t\right)$ beams during phase-matched noncollinear SWM in air. Right: A wave-vector diagram of the same process.

Figure 3

Calculated phase-matching bandwidths (black squares, right axis) for SWM in air and evaluated durations for the corresponding bandwidth-limited pulses, plotted on a logarithmic scale vs the interaction length $L$. The inset shows phase-matching bandwidth shapes for $L=3.5\text{and}2\mathrm{cm}$.

Figure 4

Numerically evaluated, according to Eqs. (2, 3, 4), generation efficiencies for the direct (blue dashed line) and SWM-assisted (red line) THG pathways. The results are normalized to maximal value at $z=1$ cm and plotted for $P_0=0.5P_{\mathrm{cr}}$ and $P_0=5P_{\mathrm{cr}}$ on the left and right panels, respectively.

Figure 5

Black squares: logarithmic plots of experimentally measured (using SWM in air) fifth-order autocorrelations from 0.6 mJ (left panel) and 0.8 mJ (right panel) pump pulses with 35 fs durations at FWHM. The blue solid line marks the recovered intensity of the pulse obtained by fitting its fifth-order correlation function (red dashed line) to the experimental data.