Higher-Order Kerr Terms Allow Ionization-Free Filamentation in Gases

We show that higher-order nonlinear indices ($n_4$, $n_6$, $n_8$, $n_{10}$) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows us to reproduce experimentally observed intensities and plasma densities in self-guided filaments.

Figure 1

(a) On-axis intensity and (b) plasma density as a function of the propagation distance for the classical model (considering only $n_2$ term of the Kerr index and the plasma defocusing), the full model, as well as the full model without plasma.

Figure 2

Fluence distribution in air as a function of the propagation distance for the full model (a) and the classical model including $n_2$, ionization and GVD only (b). The white lines display the quadratic radius as a function of the propagation distance.

Figure 3

Spectrum after 2 m propagation in air at atmospheric pressure.

Figure 4

(a) On-axis intensity and (b) plasma density as a function of the propagation distance for the classical model (considering only $n_2$ term of the Kerr index and the plasma defocusing) and the full model, in Argon under 1 bar pressure.

Figure 5

Fluence distribution in 1 bar Argon as a function of the propagation distance for the full model (a), and the classical model including $n_2$, ionization and GVD only (b).

Figure 6

Space-time dynamics of filamentation in 1 bar Argon for the full model (a), and the classical model including $n_2$, ionization and GVD only (b). Both models yield pulse splitting around 1.05 m propagation distance, but the full model where filamentation is driven by the instantaneous Kerr effect results in a more symmetrical temporal dynamics.