Zeptosecond γ-ray pulses

High-order harmonic generation (HHG) in the relativistic regime is employed to obtain zeptosecond pulses of $\gamma$-rays. The harmonics are generated from atomic systems in counterpropagating strong attosecond laser pulse trains of linear polarization. In this setup recollisions of the ionized electrons can be achieved in the highly relativistic regime via a reversal of the commonly deteriorating drift and without instability of the electron dynamics such as in a standing laser wave. As a result, coherent attosecond $\gamma$-rays in the 10 MeV energy range as well as coherent zeptosecond $\gamma$-ray pulses of MeV photon energy for time-resolved nuclear spectroscopy become feasible.