Some Fundamental Concepts of Femtosecond Laser Filamentation

This paper attempts to clarify some fundamental concepts of femtosecond (fs) laser filamentation in an optical (transparent) medium since there are currently many new applications which are being explored. These range from remote sensing in the atmosphere to few cycle pulse generation, micro-material processing, writing wave guides, fiber Bragg gratings, etc. in the laboratory. For a most recent comprehensive review, see Ref. 1. First of all, we should emphasize that the fundamental physics of filamentation is the same in all optical media, be they gases, liquids, or solids. It is simply the balancing actions between Kerr self-focusing of the pulse in a neutral medium and the self-de-focusing by the selfgenerated weak plasma (free electrons). The difference lies in the details of free electrons generation. In gases, tunnel ionization of the gas molecules inside the selffocal volume results in a plasma that defocuses the pulse [2]. In condensed matters, excitation of free electrons from the valence to the conduction bands [3] is followed by inverse Bremstrahlung and electron impact ionization [4] before the short pulse is over. The well-known type of optical breakdown of the medium (generation of a spark) by longer laser pulses in the picosecond (ps) and nanosecond (ns) regimes does not occur in the fs self-focusing regime because there is not enough time to sustain cascade (avalanche) ionization. For example, at a one-atmosphere pressure, the mean free time of an electron collision is ∼1 ps. This time is longer than the fs pulse duration so that only tunnel ionization, an ‘instantaneous’ electronic transition process, is responsible for the generation of free electrons [5] even though the full pulse is involved in the self-focusing including external