Achieving high polarization of photons emitted by unpolarized electrons in ultrastrong laser fields

Nonlinear Compton scattering driven by ultraintense lasers presents a promising avenue for enhancing the photon energy, brilliance, and setup compactness of γ-ray sources. However, a significant challenge lies in achieving a high polarization degree with commonly generated unpolarized electrons, thus addressing a longstanding puzzle in the field. Here, we investigate the polarization dynamics of photons emitted by an unpolarized electron beam interacting with a counter-propagating ultraintense laser pulse numerically and propose a method to generate highly polarized γ rays via nonlinear Compton scattering with the aid of vacuum dichroism effect. Our simulations reveal that high-brilliance γ rays with polarization beyond 90% are feasible in a single-shot interaction, rivaling the highest achieved by any γ-ray sources to date, based on a developed Monte Carlo method incorporating polarization-resolved tree processes of nonlinear Compton scattering and Breit-Wheeler pair production and one-loop vacuum polarization. This generation method showcases an extraordinary ultra-high polarization degree and a user-friendly all-optical experimental setup while harnessing the high photon energy and brilliance characteristic of nonlinear Compton scattering sources, thus making it of great potential for experimental applications.