SPATIAL CHARACTERIZATION OF LASER FILAMENTS BY DETECTION OF SIGNATURES OF IONIZATION

Abstract

Laser filamentation is a phenomenon currently being widely studied in which an ultrashort laser pulse self focuses as a result of the nonliner Kerr effect. Lim- ited data is available in terms of spatial characterization of the filament. We study the spatial distribution of molecular and atomic species generated by the filament in order to infer the relevant dynamics. We find evidence for a new impulsive vibrational excitation scheme which we introduce in this dissertation. Insight into the mechanisms of ionization is gained by consideration of the details of this process. In addition, the suitability of filaments to stimulate impulsive Raman scattering for spectroscopic purposes is evaluated. The data presented show the first measurements of ions by impulsive Raman spectroscopy. This method has advantages over previous techniques. Signal is directional and so it can be more completely collected and can be measured stand off. The energy required for detection is also provided by a probe beam rather than from the analyte molecules themselves and so there is no limit to its intensity as in the case of fluorescence spectroscopy.