Phase Control of Nonlinear Breit-Wheeler Pair Creation

High-power laser facilities produce intense laser pulses that when collided with a relativistic electron beam trigger the strong-field QED process of nonlinear Breit-Wheeler pair creation. Despite the successful generation of Breit-Wheeler pairs in past experiments, detection remains a challenge due to the low number of created pairs. In this work, a laser pulse composed of a fundamental and second harmonic is used for both Breit-Wheeler pair creation and to spatially separate the positrons from the electrons, which could facilitate experimental detection. An analytical model of the interaction between the created pairs and a plane-wave laser pulse is used to determine the phase difference and relative amplitudes of the two harmonics that optimize the positron-electron separation. Particle-in-cell simulations verify this model and show that the phase difference can indeed be used to control the separation. Aside from facilitating experimental detection of positrons without the need for large magnets, the asymmetry of a two-color electromagnetic field can also be used to spin-polarize the pairs. Combining these effects could provide an easy-to-detect polarized positron bunch.