| Popis: |
High harmonic generation (HHG) is now an accepted method for laboratory based generation of XUV and soft x-ray light. HHG produces pulses that are highly coherent, with a pulse length in the femtosecond or attosecond timescales. To predict the XUV output from an HHG experiment, both the emission from a single atom and the build-up of intensity over the interaction length must be accounted for. In addition, the propagation of the driving laser pulse in a gas-filled capillary has a significant effect on generation. To improve the understanding of these processes, this thesis presents a spatio-temporal phasematching model for capillary HHG, and a modal-based model of laser propagation in an ionising gas.The phasematching model takes into account the full spatio-temporal nature of the capillary HHG process to determine the harmonic build-up as a function of radius, time and harmonic number. A very simple single atom response is assumed, consisting of a plateau and hard cutoff. Good agreement is shown between the theoretically predicted and experimentally measured spectra for a number of gases. The model is extended to gas mixtures and shows qualitative agreement with experimental results. The spatial output of the phasematching model, together with phase information from the semi-classical model, is used to investigate the propagation of the XUV beam as it leaves the capillary. The divergence of the harmonic beam is predicted and found to be in agreement with preliminary experiments.The modal propagation model (MPM) is simple and computationally fast. The effect of misalignment of the laser into the capillary is investigated and it is shown that radial symmetry can be assumed within the capillary. Predictions of the ionisation fraction as a function of propagation distance show good agreement with experimental measurements of fluorescence.The MPM assumes that the time envelope of the pulse does not change and the plasma-induced nonlinear mode coupling is weak, and so is valid in the low pressure, low intensity regime. For extension to higher intensities and pressures, a more sophisticated model is proposed that is currently under development. |
