Generation of ultrarelativistic longitudinally spin-polarized electrons via nonlinear Compton scattering

The spin polarization of electrons in strong laser fields, an important phenomenon predicted by strong-field quantum electrodynamics, has garnered considerable attention in recent years due to its broad potential applications. In the present work we investigate the collision dynamics between multi-GeV electrons and circularly polarized laser pulses with relativistic intensity and duration of tens of femtoseconds by employing a density-matrix method. Numerical results demonstrate that the energy of outgoing electrons significantly influences their longitudinal spin polarization (LSP). Notably, for an incident electron energy of 80 GeV and a laser intensity parameter of 0.4, the LSP exceeds $60%$ around the energy corresponding to the peak of the scattering energy spectrum. Our findings provide a promising avenue for experimentally generating ultrarelativistic longitudinally spin-polarized electron beams.