Methods for increased energy and flux in high harmonic generation

High harmonic generation (HHG) is a nonlinear light matter interaction that results in the generation of high order harmonics of a driving optical field. It is routinely used to generate coherent short wavelength radiation in the soft x-ray and extreme ultraviolet (XUV) regimes. HHG-based XUV sources require a highly intense driving pulse to be focused into a target gas typically within a gas cell, gas jet or hollow capillary. They can be used for a variety of applications, one of which is nanoscale imaging. The work presented in this thesis focuses on the development of two high flux HHG sources for use in tabletop nanoscale imaging; a capillary based HHG system using a Ti:Sapphire based laser and a gas cell based HHG system using an Yb-doped fibre laser.The manufacture and use of a 7 cm hollow core capillary in HHG is described. The propagation of the pump pulse is modelled using a new nonlinear propagation model and compared to experimental results. The pulse is found to undergo pulse self-compression using a new regime of high ionisation pulse compression. The pulse is observed to reduce in length from 53 fs to 28 fs, with post compression reducing this further to 15 fs. The XUV spectrum from the 7 cm capillary is measured and its dependence on gas pressure discussed using calculations of the XUV transmission within the capillary. Using the observations made of the 7 cm capillary a new more efficient 4.5 cm capillary is designed and manufactured. Comparison between the two capillaries shows an increase in flux of the new capillary design of more than an order of magnitude, with a calculated value of 5.3 x 1012 ph harm-1 s-1 cm-2, one of the highest in the world.A gas cell is used in the Yb-doped fibre laser based HHG source and the XUV signal is measured using an XUV photodiode. The XUV signal is characterised by measuring its dependence on focal position, gas pressure and pump laser power. A novel method of increasing the flux by twisting of a second lens outside the vacuum chamber is discovered and was found to double the measured signal. The maximum flux for this fibre laser based HHG source is calculated and found to be 2.2 x 1012 ph s-1, the highest measured for a fibre based HHG source