Self-Trapping of Supercontinuum Generated by Femtosecond Pulses in a Noble Gas

INTRODUCTION. The observation of ultra-broadband supercontinuum generation in gases was first reported by Corkum et al. [1]. The possibility to produce high energy white-light continua (WLC) using ultra-short pulses has sparked interest in applications such as remote sensing in the atmosphere [2] and nonlinear spectroscopy [3]. In contrast to white-light generation in solid-state fibers, the nature of WLC generation in gases is not fully understood. The opportunity of having a broadband source of radiation confined in one beam with high spectral energy density has paved the way for replacing conventional sources of radiation such as optical parametric generators/amplifiers (OPGs/OPAs) [3]. The use of parametric devices requires phase matching of nonlinear crystals to generate and amplify certain frequencies which lead to the repeated realignment of optical experiments. In addition, the importance of having good spatial quality beams for nonlinear optical experiments often requires spatial filtering. The spatial properties of WLC from gases have been shown to be excellent over most of their bandwidth. Thus, having a single white-light filament as a tunable source requires only the use of narrow band-pass filters in order to perform nonlinear spectroscopic characterization of materials [3]. Based on this, there is an ongoing effort to understand the mechanisms and limitations involved in producing a high energy ultra-broadband source of radiation. In this work, we have investigated the two main nonlinear mechanisms, namely self phase modulation and the production of plasma, that are thought to be involved in the generation of a stable WLC in gases in order to achieve high spectral energy output. The calculations presented in the current work confirm the extent of the nonlinear index change due to the plasma production as well as the conditions necessary to attain a self guided filament through self focusing of femtosecond optical pulses in a pressurized noble gas.