When a femtosecond laser pulse propagates in transparent media, a fascinating phenomenon called filamentation can appear if the power of the input pulse is larger than the critical power Pcr（=3.77/8n0n2）for self-focusing, where 0 is the laser wavelength and n0 and n2 are the linear refractive index and Kerr nonlinear coefficient, respectively. Generally, this phenomenon is believed to stem from the dynamic balance of the Kerr self-focusing effect and defocusing effect due to plasma generation (the so-called standard Kerr model).

Researchers at State Key Laboratory of High Field Laser Physics—Shanghai Institute of Optics and Fines Mechanics (SIOM/China) and Xuzhou Normal University numerically investigate this question from the viewpoint of spatiotemporal dynamics. They find that some essential differences between the two filamentation models can be obtained, which, in their opinion, is enough to distinguish these two kinds of defocusing effects.[ OPTICS LETTERS / Vol. 36, No. 12 / June 15, 2011]

In summary, They numerically show the difference in spatiotemporal dynamics of femtosecond filamentation in argon between the two filamentation models. When the higher-order Kerr effect model is used, the on-axis temporal profile should be approximately super-Gaussian and the pulse splitting and pulse self-compression effect cannot appear, which is inconsistent with some previous experimental results. They infer that the higher-order nonlinear non-linear coefficients of argon are overestimated and plasma should be the defocusing mechanism at least in the argon filament.