Optimisation and Application of Intense High-Order Harmonic Pulses

Abstract: The high-order harmonics that are created in the interaction between an intense laser pulse (around 10¹⁴ W/cm²) and a gas medium are studied. The high-order harmonic radiation is a relatively new source of coherent radiation in the short wavelength range. The high-order harmonics are interesting for applications because of their good spatial and temporal coherence, their short pulse duration and the possibility to focus the harmonic radiation to high intensity. The first part of this thesis focuses on optimising the high-order harmonic pulse energy. This is motivated by the relatively low conversion efficiency of high-order harmonic generation. These studies are aimed to understand how the conversion efficiency can be improved. The harmonic yield as a function of medium length is studied and the different effects that influence the harmonic yield are discussed. A new geometrical configuration with two collinear foci along the propagation axis is found to improve the harmonic conversion efficiency compared to an ordinary one-focus beam. In addition, theoretical optimisations of the harmonic pulse energy are carried out. With a genetic optimisation algorithm, two or three parameters related to the characteristics of the laser beam and the nonlinear medium are varied simultaneously. Over 10⁹ photons per pulse at a wavelength of 28 nm can be obtained. The underlying idea of the second part of this thesis is that the high-order harmonics are potentially suitable for generating pulses of shorter duration than what is possible with current technology. With the genetic algorithm, the temporal properties of the harmonic pulse are optimised theoretically. Harmonic pulse durations as short as 6 fs are obtained, generated by a 50 fs laser pulse. The temporal coherence is optimised by selecting the transform-limited part of the harmonic spectrum. In this condition, the time profile of the harmonic radiation is a train of 265 attosecond long pulses. By experimentally manipulating the ellipticity of the laser pulse in time, one or two temporal gates of a few femtosecond duration can be created where the polarisation is linear and harmonic emission is efficient. With this method, the frequency chirp of the harmonics is studied. The fifth harmonic beam (160 nm) is used to experimentally induce nonlinear multiphoton ionisation in the rare gases. This might enable measurement of ultra-short harmonic pulse durations at short wavelengths with an auto-correlation method.