Frontiers of High-Harmonic Generation

High-harmonic generation (HHG) is investigated theoretically aiming to improve its fundamental understanding and overcome current limitations. In the course of this, HHG is extended to higher energies and used to generate x-ray pulses with durations down to the zeptosecond regime. To this end, a route is discussed to generate multi-kilo-electronvolt harmonics with relativistic laser intensities circumventing both the phase-matching problem and relativistic drift. In a different approach, the state-of-the art attosecond science is transferred to the x-ray regime by manipulating the atom during the excursion of the continuum electron. By coherently exciting a core electron, the continuum electron is forced to recombine with the core hole with a much higher binding energy. HHG is the cutting-edge technology allowing for the creation of the shortest coherent light flashes of all. However, particularly problematic is the intrinsic chirp of the HHG light that requires the use of optical elements to compensate it. For this purpose, the harmonic generation process is altered such that chirp-free pulses are generated with possible pulse duration below one attosecond. Moreover, in a more fundamental work, the HHG process is set into the framework of a unified interference picture. In that picture, all possible HHG transitions can be viewed as an interference between two wave packets in the binding potential leading to photoemission at their difference energy. A novel continuum-continuum transition is studied and its importance in the saturation regime is pointed out