On negative higher-order Kerr effect and filamentation

Abstract

As a contribution to the ongoing controversy about the role of higher-order Kerr effect (HOKE) in laser filamentation, we first provide thorough details about the protocol that has been employed to infer the HOKE indices from the experiment. Next, we discuss potential sources of artifact in the experimental measurements of these terms and show that neither the value of the observed birefringence, nor its inversion, nor the intensity at which it is observed, appear to be flawed. Furthermore, we argue that, independently on our values, the principle of including HOKE is straightforward. Due to the different temporal and spectral dynamics, the respective efficiency of defocusing by the plasma and by the HOKE is expected to depend substantially on both incident wavelength and pulse duration. The discussion should therefore focus on defining the conditions where each filamentation regime dominates.

Interpretation of negative birefringence observed in strong-field optical pump-probe experiments: High-order Kerr and plasma grating effects

“…This is consistent with single-color pumpprobe experiment, which exhibits refractive index saturation and inversion [1,5,43], and two-color experiments using non overlapping spectra that do not [26,43,44]. However, the results presented in this section show that the sign inversion can also be produced with two-color non-overlapping spectra provided that much larger intensity is used compared to the one-color scheme.…”

“…The potential impact of the HOK model on the last process was presented in Refs. [14] and [5]. These works reveal that Kerr saturation due to high-order terms can lead under certain conditions to a self-guided nonlinear pulse propagation without requiring the need for a plasma production.…”

“…Finally, it should be pointed that large chirps, as those responsible for symmetric PG signal [42], lead at constant energy level to a dramatic drop of intensity and consequently to a significant decrease of the birefringence signal above the inversion threshold. This along with a temporal broadening of the signal could have been straightly noticed in the previous studies [1,5] so that any unforeseen significant chirp would have been corrected.…”

“…In order to avoid significant nonlinear propagation effects, like intensity clamping, the gas pressure in the static cell was reduced to a low value reconcilable with an acceptable signal-to-noise ratio. As in the previous experiments [1,5], the compression of the laser pulses at the interaction region was checked by maximizing the signal. The result obtained at 30 µJ [ Fig.…”

“…We attributed the transition between these linear and nonlinear regimes to a saturation of the electronic Kerr effect induced by the intense short laser pulse. Based on different assumptions, we proposed a phenomenological model by introducing in the standard definition of the nonlinear refractive index n Kerr = n 2 I, where n 2 is the Kerr index and I is the applied intensity [4], high-order Kerr (HOK) terms n 4 , n 6 , n 8 , and eventually n 10 [1,2,5]. Starting from the weak field regime, each term was successively fitted to the experiment by gradually increasing the intensity so as to minimize cross correlations.…”