Final Activity Report Summary - HIGH-FIELD OPCPA (Monocycle multi-terawatt laser pulses for the generation of ultrashort, ultrabright electron and X-ray bunches) The quest for understanding and controlling microscopic processes at a fundamental level has been driving the development of tools that allow for temporal and spatial resolutions on the order of the characteristic time and length scales of these processes. The dynamics of the electron motion within atoms and molecules govern any such process at the most fundamental level. The corresponding time scales are typically of a few femtoseconds or even down to the attosecond range, while the characteristic spatial separations are around 1Angstrom. Both these spatial and temporal resolutions have been achieved separately in recent experiments, however, to date a full 4D investigation on these scales has not been possible due to the lack of suitable infrastructures. The desired tools for such investigations in the ground state as well as excited states would consist of Angstrom-wavelength X-ray and electron pulses with sub-fs duration that can be used for both probing the processes and controlling them by inducing the appropriate excitation.It has recently been demonstrated that the interaction of an ultrashort, high-power laser pulse with a plasma can lead to the generation of accelerated electron bunches displaying a narrow energy distribution. Such electron beams would represent an ideal driver for a table top X-ray free electron laser. As predicted by theory, the electron acceleration process during the high-power laser-plasma interaction could be driven even more efficiently and in a more stable fashion, if ultra-high power laser pulses (i.e. on the PW scale) with a duration of only a few cycles of the light field were used.The project "Monocycle multi-terawatt laser pulses for the generation of untrashort, ultrabright electron and X-ray bunches" aimed at both, developing such a novel light source for this application and setting up the an electron-acceleration experiment using pulses from an available TW-scale laser system at MPQ. In contrast to the conventional solid-state laser systems which are limited to pulse durations of several tens of femtoseconds, the feasibility of optical parametric chirped pulse amplification (OPCPA), when pumped with very short pulses, was tested. We found that ultra broadband amplification was possible in such a setup, supporting very short pulse durations, which was also demonstrated in the compression experiments. However, the energy scalability of this approach remains to be shown and is currently under development. The outcomes of the laser development part of the present project can directly be applied to a large-scale project, the Petawatt Field Synthesiser (PFS) at the Max-Planck-Institut für Quantenoptik in Garching, which is also based on this modified OPCPA scheme. This represents a major step towards the construction of a petawatt-scale, few-cycle light infrastructure.On the other hand electron-acceleration experiments were setup and performed at the MPQ using the existing ATLAS laser facility. We succeeded in generating GeV-scale electron energies when a gas-filled capillary discharge waveguide was used in order to guide the laser pulse and thereby aid the acceleration process. In addition, a stable regime of electron acceleration was identified, which has not been reported before. Reproducibility and stability of the laser-accelerated electron bunches constitute a crucial prerequisite when considering further applications such as driving an X-ray free electron laser.In summary, during the course of the project important advances have been achieved in both the laser development and the investigation of the high-power laser-plasma interaction processes. Further development is ongoing in both areas which will eventually come together on the route towards achieving the long-term goals.