Generation of High-Brilliance Polarized γ-Rays Via Vacuum Dichroism-Assisted Vacuum Birefringence

High-brilliance circularly polarized γ-photon beams are of great significance for a wide range of applications. However, their generation through nonlinear Compton scattering must require a high-density longitudinally-spin-polarized electron beam and consequently is still a great challenge. Here, a novel method is proposed to generate such γ-photon beams via the vacuum dichroism (VD)-assisted vacuum birefringence (VB) effect, only utilizing a well-established unpolarized electron beam. A linearly polarized laser pulse is splitted into two subpulses with the first one colliding with a dense unpolarized electron beam to generate a linearly polarized γ-photon beam (via nonlinear Compton scattering), which then further collides with the second subpulse and is transformed into a circularly polarized one via the VB effect. It is found that by manipulating the relative polarization of two subpulses, one can "purify" the polarization of the γ-photon beam via the VD effect, thereby significantly enhancing the circular polarization of the γ-photon beam. Due to the VD assistance, the VB effect reaches optimal when the relative polarization is nearly 30°, not the widely used 45° in the common VB detection methods. The numerical results show that one can obtain a circularly polarized γ-photon beam with average degree of about 30% (43%) for energies above 500 (1000) MeV and brilliance of about 10²⁴ (10²³) photons/(s · mm² · mrad² · 0.1%BW) at 500 (1000) MeV by using a currently feasible laser with a peak intensity of about 10²² Wcm^-2. And, it can be further improved to above 60% (75%) by increasing the laser pulse duration. Moreover, our method is shown to be robust with respect to the laser and electron beam parameters, and can also be used to efficiently confirm the well-known VB effect itself, which has been predicted a very long time ago but has not been directly observed in experiments yet.