Ultraintense half-cycle zeptosecond pulse generation via cascaded coherent wakefield synchrotron emission

We report a unique regime for generating zeptosecond pulses (ZPs) to break the attosecond timescale barrier through cascaded coherent synchrotron emission driven by ultraintense x-ray laser-pulse-excited wakefields in foil. When an x-ray laser pulse interacts with the near-critical-density but exactly overdense plasma, it excites huge wakefields with huge accelerating gradients, while the displaced electrons are shaped into several electron nanosheets. The primary relativistic electron sheet is accelerated by the wakefields and emits coherent radiation with a long electromagnetic field tail. The subsquent electron sheet is longitudinally accelerated as well as transversely perturbed by the radiation tail and emits another pulse. This cascaded process is repeated several times and finally results in ZP generation. This cascaded coherent wakefield synchrotron emission regime establishes a viable pathway for accessing light-matter interactions in the zeptosecond regime, probing vacuum quantum electrodynamics and subnucleonic scale dynamics.