Investigations of short-pulse ultraviolet laser interactions with gas targets

Thesis

This thesis presents investigations of interactions of a 380 fs, 248.6 nm KrF laser beam focused to intensities up to 10¹⁸ W/cm² with gas targets of Helium and Neon. At these intensities, the targets are rapidly ionised by the process of optical field ionisation. The purpose of these investigations is two-fold. Firstly, we determine the relative importance of plasma heating mechanisms and investigate the scaling of electron temperature with density to ascertain whether an optically-ionised XUV recombination laser, utilising the 3s-2p transition in a Lithium-like ion, is viable. Such a laser requires, for the case of Neon, the production of very cold (<50 eV) electron temperatures at electron densities of the order of 5 x 10²⁰ cm^-3 , and optical-ionisation to the Helium-like stage of Neon (Ne⁸⁺). The results are compared and contrasted to computer simulations of the electron temperature. Secondly, we investigate the process of harmonic generation. We show that, contrary to commonly held beliefs, it is possible to generate shorter harmonic wavelengths from ions using a short wavelength laser than from atoms using longer wavelengths. We also show high harmonic generation efficiencies from neutral Helium at lower intensities (~10¹⁵ W/cm²) away from the laser focus. The results are compared to, and found to be matched well by, simulations which predict the single atom/ion harmonic response.

Authors: Preston, S

• DOI: 10.5287/ora-7qbxxqa5b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford