The Influence of Electron Plasma Formation on Superbroadening in Light Filaments

The extensive lit’erature on theoretical and experimental investigations of superbroadening in small scale filaments produced by self-focussed light pulses has been reviewed by Akhmanov et al. [ 11. The interpretation of this frequency broadening in terms of moving focal spots has been reviewed more recently by Low and Shen [2]. In all of this work the diameter of the filaments, or the size of the focal spot, is introduced as a phenomenological parameter which is chosen to conform with experimental observations. Yablonovitch and Bloembergen [3] have demonstrated that the maximum power flux density in a filament or focal spot, even one with a duration of a picosecond, is limited by the onset of dielectric breakdown of the optically transparent medium. The formation of an electron plasma with an electron density N,(t) and an electronic collision time 7 leads to a contribution to the complex index of refraction