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Hörlein, Rainer (2009): Investigation of the XUV Emission from the Interaction of Intense Femtosecond Laser Pulses with Solid Targets. Dissertation, LMU München: Faculty of Physics
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Abstract

The generation of coherent high-order harmonics from the interaction of ultra-intense femtosecond laser pulses with solid density plasmas holds the promise for table-top sources of intense extreme ultraviolet (XUV) and soft x-ray (SXR) radiation. Furthermore, they give rise to the prospect of combining the attosecond pulse duration of conventional gas-harmonic sources with the photon flux currently only available from large-scale free-electron laser or synchrotron facilities. In this thesis a series of experiments studying various aspects of harmonic generation from such a plasma source are presented and the emitted XUV-radiation is characterized spectrally, spatially and temporally. The measurements probe the dynamics of the plasma surface on a sub-laser-cycle time scale and help to increase our understanding of the harmonic generation process. It is shown that, at moderate intensities and laser contrast, the emitted harmonics are indeed phase-locked but chirped and emitted as a train of XUV-bursts of attosecond duration. Measurements with very high contrast relativistically intense driving pulses reveal the generation of harmonics up to the relativistic cutoff in a diffraction-limited beam with constant divergence observed for all wavelength. This implies that the harmonics are generated on a curved surface and travel through a focus after the target possibly opening a route towards extreme intensities in the process. In addition it is found that a target roughness on the scale of the wavelength of the highest generated harmonic does not adversely affect the harmonic beam quality implying that the generation of diffraction-limited keV-harmonic beams should be possible. In a third set of experiments the first demonstration of harmonic generation from solid targets using an 8 fs driving laser opens a route towards the generation of ultra-intense single-as pulses and gives conclusive evidence for the unequal spacing of the harmonic emission. Based on these results the development of ultra-intense sources of single as-pulses from the interaction of intense laser pulses with solid surfaces could advance at a fast pace making XUV-pump XUV-probe type investigations of nonlinear processes with attosecond time resolution feasible in the near future.