Electron spin polarization around magnetic nodes under the chirped rotating electric field

Electron spin polarization in an intense laser field presents a promising avenue for generating highly polarized high-energy electron sources. In this study, we investigate numerically the spin polarization dynamics of an electron in a rotating laser field with a frequency chirp. We consider a standing wave formed by two counterpropagating chirped laser pulses, which have different chirp modes: linear and time-reversed chirp. Our results reveal that the frequency chirp breaks the time symmetry of the field, thereby altering the laser-electron interaction dynamics. Consequently, an electron can gain more net energy from a chirped laser field through optimal phase synchronization between the electron and the laser field. This results in a higher electron quantum parameter and an increased photon emission probability. As a result, the probability of spin flip for an electron during radiation in the magnetic node plane is enhanced, leading to improved electron polarization efficiency. We also find that the polarization degree is highly sensitive to the chirp parameter, and we identify an optimal chirp value that maximizes the polarization degree. By applying this optimal chirp parameter in the time-reversed chirped case, the electron's relative polarization can be enhanced by $20%$ and reach its maximum value of approximately $85%$.