Defocusing role in femtosecond filamentation: Higher-order Kerr effect or plasma effect?**Project supported by the National Basic Research Program of China (Grant No. 2013CB922200), the National Natural Science Foundation of China (Grant Nos. 11034003 and 11474129), the Research Fund for the Doctoral Program of Higher Education in China (Grant No. 20130061110021), and the Graduate Innovation Fund of Jilin University, China (Grant

Abstract

The femtosecond filamentation in the classical and high-order Kerr (HOK) models is numerically investigated by adopting multi-photon ionization (MPI) cross section with different values. It is found that in the case that the MPI cross section is relatively small, there exists a big difference between the electron density as well as clamped intensity calculated in the classical model and those calculated in the HOK one, while in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated in the two models are nearly in agreement with each other, and under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provides an approach to determine which effect plays the dominant defocusing role. These results demonstrate that it is ionization that results in the difference between the two models.