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b'The investigation of laser-matter interactions calls for ever shorter pulses as new\\neffects can thus be explored. With laser pulses consisting of only a few cycles of the\\nelectric field, the phase of these electric field oscillations becomes important for many\\napplications.\\nIn this thesis ultrafast laser sources are presented that provide few-cycle laser pulses\\nwith controlled evolution of the electric field waveform. Firstly, a technique for phasestabilizing\\nultra-broadband oscillators is discussed. With a simple setup it improves the\\nreproducibility of the phase by an order of magnitude compared to previously existing\\nmethods.\\nIn a further step, such a phase-stabilized oscillator was integrated into a chirped-pulse\\namplifier. The preservation of phase-stability during amplification is ensured by\\nsecondary phase detection. The phase-stabilized intense laser pulses from this system\\nwere employed in a series of experiments that studied strong-field phenomena in a\\ntime-resolved manner. For instance, the laser-induced tunneling of electrons from\\natoms was studied on a sub-femtosecond timescale.\\nAdditional evidence for the reproducibility of the electric field waveform of the laser\\npulses is presented here: individual signatures of the electric field half-cycles were\\nfound in photoelectron spectra from above-threshold ionization.\\nFrequency conversion of intense laser pulses by high-order harmonic generation is a\\ncommon way of producing coherent light in the extreme ultraviolet (XUV) spectral\\nregion. Many attempts have been made to increase the low efficiency of this nonlinear\\nprocess, e.g. by quasi phase-matching. Here, high-harmonic generation from solid\\nsurfaces under grazing incidence instead from a gas target is studied as higher\\nefficiencies are expected in this configuration.\\nAnother approach to increasing the efficiency of high-harmonic generation is the\\nplacing of the gas target in an enhancement resonator. Additionally, the production of\\nXUV photons happens at the full repetition rate of the seeding laser, i.e. in the region\\nof several tens to hundreds of megahertz. This high repetition rate enables the use of\\nthe XUV light for high-precision optical frequency metrology with the frequency comb\\ntechnique. With such an arrangement, harmonics up to 15th order were produced. A\\nbuild-up cavity that stacks femtosecond laser pulses in a coherent manner to produce\\nintra-cavity pulse energies of more than ten microjoules at a repetition rate of ten\\nmegahertz is presented here.\\nWith this high average power measuring hitherto uninvestigated optical transition\\nfrequencies in the XUV, such as the 1S-2S transition in singly charged helium ions\\nmay become a reality.'