Highly polarized positrons generated via few-PW lasers

Spin-polarized positron beams have widely been utilized in applications ranging from fundamental physical studies to material processing. Preparing highly polarized positron beams for accurate probing is a long-standing issue. Here, we put forward a method to produce ultra-relativistic polarized positrons with unprecedented purity in a femtosecond timescale employing a few-PW circularly polarized laser pulse. The fully spin-resolved QED Monte Carlo method is used for simulating the two successive QED processes during the interaction, i.e., nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. As the photons emitted in a circularly polarized laser field are symmetrically polarized, the polarization of the intermediate gamma photon beam averages out to zero, which is advantageous for improving the polarization of positrons. Meanwhile, the moderate laser intensity suppresses the depolarization of the new-born positrons induced by radiation reaction effect. As a result, the polarization of the positrons can reach up to = 90%, the highest among the laser-driven polarization schemes conceived hitherto. Furthermore, our method relaxes the requirement on laser intensity to few-PW level, offering a promising way of preparing polarized positrons with current-generation laser facilities.