A novel scheme allowing for relativistic collisions of laser-accelerated electrons is introduced. Two spatially separated electron bunches are driven in opposite directions by two counterpropagating laser pulses until they reach the point of collision which lies within the laser fields. This method can be employed to accelerate electrons to the maximum kinetic energy which can be transferred to charged particles by plane propagating laser fields. Due to the symmetric setup, the center of momentum is at rest with respect to the laser propagation direction such that virtually the whole kinetic energy is available for particle reactions.

The important process of laser-driven recollisions, where electrons are accelerated by strong laser fields and return to their parent ions, breaks down if the laser intensities imply relativistic electron dynamics. In this case, the Lorentz force drags the electrons away in the laser propagation direction, which inhibits recollisions. Here, a variety of schemes are discussed and compared which generalize the concept of recollisions to the relativistic regime. Additional static electric fields, antisymmetric initial states, and tailored laser pulses are suitable for weakly to moderately relativistic energies, whereas standing waves, preaccelerated ions, positronium, and counterpropagating consecutive pulses allow for recollisions up to the highly relativistic regime.

It is demonstrated that a magnetic field pulse can be applied to refocus a spreading electron wave packet in a relativistic recollision scheme. The drift due to the Lorentz force occurring in strong laser fields is employed to separate an electron from the core. Before it is driven back for recollision by a second counter-propagating laser pulse, the dynamics of wave packet spreading is reversed by a magnetic field pulse. At the instant of recollision, the electron wave packet is refocussed to small spatial widths while it holds maximal kinetic energy. This way, efficient recollisions are shown to occur up to the high-energy regime.

The dynamics of an electron in crossed laser fields is investigated analytically. Two different standing wave configurations are compared. The counterpropagating laser waves are either linearly or circularly polarized. Both configurations have in common that there are one-dimensional trajectories on which the electron can oscillate with vanishing Lorentz force. The dynamics is analyzed for the situations when the electron moves in the vicinity of these ideal axes. If the laser intensities imply nonrelativistic electron dynamics, the system is described quantum mechanically. A semiclassical treatment renders the strongly relativistic regime accessible as well. To describe relativistic wave packets, the results of the classical analysis are employed for a Monte Carlo ensemble. This allows for a comparison of the wave packet dynamics for both configurations in the strongly relativistic regime. It is found for certain cases that relativity slows down the dynamics, i.e., for higher laser intensities, wave packet spreading and the drift away from the ideal axis of vanishing Lorentz force are shown to be increasingly suppressed.

A recollision scheme for laser-driven electrons is introduced, which allows for electron-core collisions with relativistic energies up to the GeV-regime. The drift velocity occurring in strong laser fields is employed to drive electrons first away from the core and then back for recollisions. These two steps are driven by two consecutive, counter-propagating laser pulses originating most favourably from the same source. The intensities of the pulses are chosen in a way to enable recollisions with the maximal kinetic energy which a laser-driven electron can reach in a propagating laser field.

An analytical approach to quantum-mechanical wave-packet dynamics of laser-driven particles is presented. The time-dependent Schrodinger equation is solved for an electron exposed to a linearly polarized plane wave of arbitrary shape. The calculation goes beyond the dipole approximation, such that magnetic field effects like wave-packet shearing are included. Analytical expressions for the time-dependent widths of the wave packet and its orientation are established. These allow for a simple understanding of the wave-packet dynamics.