Investigations of high harmonic generation in laser induced ablation plumes

• Main Author: Abdelrahman, Zara
• Other Authors: Tisch, John
• Published: Imperial College London 2015
• Subjects: 535
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.689093

High harmonic generation (HHG) in gases provides a compact and versatile source of coherent light in the extreme ultraviolet and soft X-ray range. It is also used to produce attosecond pulses. This thesis investigates ablation plumes as an alternative medium to gases for HHG. Of particular interest are the resonantly enhanced harmonics seen from certain plumes. Resonant HHG mechanisms and emission were investigated on the microscopic and macroscopic scale. Experiments were carried out in zinc and indium plumes, using few-cycle femtosecond HHG driving pulses. These included observing the effects of varying: carrier envelope phase (CEP), pulse duration, chirp, bandwidth and polarisation of the driving field. These results were compared to theoretical simulations, using the Four Step Model. The experimental results showed strong resonant harmonics in both zinc, at 19.1 eV and indium, at 19.7 eV, 20.5 eV and 22.3 eV. These were in good agreement with known autoionising-ground state transitions thought to cause the enhancements. The harmonics were dependent on the driving beam ellipticity, showing free election mechanisms are involved. They were found to be weakly CEP dependent, but sensitive to the peak intensity of the driving IR field. Using Young's slit diffraction patterns, direct measurements of the spatial coherent properties were made. The fringe visibility, V , measured was high, proving good spatial coherence with: VZn = 0.74 and VIn = 0.66. These were better than that measured from argon gas, VAr = 0.47, under similar experimental conditions. The first attempts to use ablation plumes to produce gas phase targets from a solid were carried out, using uracil and thymine targets. Harmonics were seen in uracil however, not from thymine. Time of flight and debris analysis of the plume attributed this to fragmentation of the thymine molecules.