Investigation of nonlinear dynamics in and via femtosecond filaments in gases

Abstract

Intense, ultrashort laser pulses are required for the study of many nonlinear optical effects and are of utmost relevance for various applications from ultrafast X-ray radiography in medicine up to remote sensing of the atmosphere. Increasing their intensity while interacting with matter eventually leads to the generation of laser-induced plasma. This plasma has fascinating optical properties such as a negative refractive index contribution proportional to the free electron density or the lack of a damage threshold, giving the prospect of a multitude of new applications based on the manipulation of light with plasma. The realization of such plasma-based applications requires a precise knowledge of its properties and temporal evolution, as the plasma remains for much longer than its generation event. A method to generate and investigate ultrashort laser pulses as well as laser-induced plasma is femtosecond filamentation. It represents the formation of an intense self-guided light channel in a medium for distances much longer than the Rayleigh range of the same beam focused in vacuum. It is formed by a dynamic balance of Kerr-induced self-focusing and plasma-induced defocusing. In this thesis, it is demonstrated that femtosecond filamentation can be employed as a tool to investigate the temporal evolution of laser-induced plasma. The study is realized in various atomic and molecular gas atmospheres via measuring the temporal evolution of the enhancement of third harmonic radiation generated by a femtosecond filament which is intercepted by a laser-induced plasma spot. Significant differences for the lifetime of the plasma in atomic and molecular gas atmospheres are found. Further, a novel method for the complete spatio-temporal characterization of a femtosecond filament along its length is presented. It is based on controlled filament termination at various positions along its length in combination with spatio-temporal pulse characterization and numerical backpropagation of the filament pulses to the termination point. The capabilities of the method are illustrated by revealing complex spatio-temporal dynamics and couplings during filament propagation.