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Nomura, Yutaka (2008): Temporal characterization of harmonic radiation generated by intense laser-plasma interaction. Dissertation, LMU München: Fakultät für Physik



Attosecond physics has become one of the most thriving field of science over the last decade. Although high-order harmonic generation from gaseous media is widely used as the source of attosecond pulses, a demand for more intense coherent extreme ultraviolet (XUV) and soft x-ray (SXR) radiation sources is growing. The process of high-order harmonic generation from plasma surfaces has attracted a strong interest as a promising candidate to meet this demand. Despite many theoretical predictions of the possibilities to generate intense attosecond pulses, experimental verifications are yet to come. The main theme of this thesis is to characterize the temporal structure of the harmonics generated from plasma surfaces. To achieve this goal, several preparatory experiments are made first. The contrast of the laser pulse is one of the most critical parameters for the harmonic generation process and its improvement is demonstrated by using a plasma mirror. The properties of the generated harmonics are studied thoroughly to find the optimal condition for temporal characterization. These experiments provide the groundwork for the autocorrelation measurements of the pulse train. To characterize the temporal structure of the generated harmonics, the technique of the volume autocorrelation using two-photon ionization of helium is applied. The measured autocorrelation traces reveal attosecond structures within the XUV radiation generated from the plasma surfaces for the first time. The observation of attosecond structures prove the potential of this harmonic generation process as a source of attosecond pulses. The process holds a promise to generate attosecond pulses with unprecedented intensities, which will open up a new regime of attosecond physics.