Non-linear attosecond physics at 100 eV

Abstract: Non-linear interactions between light and matter have nowadays a broad range of applications. They are used for frequency doubling in simple laser pointers as well as for a variety of purposes in complex laser systems like the one presented in this thesis. For the study of ultrafast phenomena, those non-linear interactions are crucial to trigger and observe events at the fastest timescale, which is currently the attosecond regime (10-15 – 10-18 s). As the duration of a single optical cycle of a visible light wave is longer than this timescale, these investigations necessitate the application of XUV and X-ray pulses. However, the generation of isolated attosecond light pulses sufficiently intense to initiate non-linear interactions with matter is restricted to photon energies below 50 eV. The aim of this thesis is to establish a new light source, which pushes this boundary further and thereby enables the observation of up to now unrevealed electron dynamics.The presented new light source provides attosecond pulses with approximately hundred times more pulse energy than typical systems (up to 55 nJ in the spectral range from approximately 65 eV to 140 eV). This facilitates non-linear measurements at these photon energies. The achieved high energy stability (5 %) of this light source allows precise and time efficient measurements. These parameters are obtained via energy-upscaling of high-harmonic generation in gas medium. For the generation of well isolated attosecond pulses a unique laser, like the Light Wave Synthesizer 20, is necessary. This laser uses optical parametric synthesis to produce the most intense sub-5 fs, sub two-cycle laser pulses in the world (80 mJ, 4.5 fs).Furthermore, an optimal focus of the XUV pulses is crucial to provide the necessary intensity for non-linear interactions. Therefore, different methods for focusing the XUV pulses are investigated. Moreover, the construction and characterization of a robust split and delay stage is presented, which is essential for time resolved measurements.The detection of the non-linear interaction is realized via a spatially resolved ion time-of-flight detector, the ion microscope. This allows for a quantitative measurement of different ionization states. With the combination of this detector and the new light source the non-linear generation of Xe4+ and Xe5+ at photon energies around 100 eV is demonstrated. This enables the determination of the two-photon ionization cross-sections, which could up to now only be measured with much longer pulses at large scientific infrastructures. This paves the way towards time-resolved XUV pump – XUV probe measurements at 100 eV.