Transition from Plasma-Driven to Kerr-Driven Laser Filamentation

While filaments are generally interpreted as a dynamic balance between Kerr focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE) is actively debated as a potentially dominant defocusing contribution to filament stabilization. In a pump-probe experiment supported by numerical simulations, we demonstrate the transition between two distinct filamentation regimes at 800 nm. For long pulses (1.2 ps), the plasma substantially contributes to filamentation, while this contribution vanishes for short pulses (70 fs). These results confirm the occurrence, in adequate conditions, of filamentation driven by the HOKE rather than by plasma.

Figure 1

Experimental setup. Two orthogonally polarized identical pulses with an adjustable delay are focused in an argon-filled cell. At the output of the cell, the probe spectrum is selected by a Glan cube polarizer and analyzed with a spectrometer.

Figure 2

(a),(b) Experimental spectra of the filaments generated by a long (1.2 ps, a) and short (70 fs, b) pulse conveying 2.5 critical powers, both with and without a pump pulse $\tau =2\mathrm{ps}$ and $\tau =1\mathrm{ps}$ ahead of it, respectively. This delay ensures that the pump pulse can only influence the probe pulse through the defocusing free electrons left behind by the pump. The input spectra are also given for reference. (c)–(d) Contrast between the spectra with and without pump pulse. The probe pulse is affected by the plasma left by the pump pulse only in the long-pulse regime.

Figure 3

(a),(b) Spectra of the filaments generated by the long (a) and short (b) pulse, simulated by the full numerical model considering the HOKE. (c–d) Contrast between the spectra with and without pump pulse. The probe pulse is affected by the plasma left by the pump pulse only in the long-pulse regime.

Figure 4

(a),(b) Spectra of the filaments generated by the long (a) and short (b) pulse, simulated by the truncated numerical model disregarding the HOKE. (c)–(d) Contrast between the spectra with and without pump pulse. The probe pulse is affected by the plasma left by the pump pulse whatever the pulse duration.