Ultra-bright γ-ray emission and dense positron production from two laser-driven colliding foils

Matter can be transferred into energy and the opposite transformation is also possible by use of high-power lasers. A laser pulse in plasma can convert its energy into γ-rays and then e ^- e ⁺ pairs via the multi-photon Breit-Wheeler process. Production of dense positrons at GeV energies is very challenging since extremely high laser intensity ~10²⁴ Wcm^-2 is required. Here we propose an all-optical scheme for ultra-bright γ-ray emission and dense positron production with lasers at intensity of 10^22-23 Wcm^-2. By irradiating two colliding elliptically-polarized lasers onto two diamondlike carbon foils, electrons in the focal region of one foil are rapidly accelerated by the laser radiation pressure and interact with the other intense laser pulse which penetrates through the second foil due to relativistically induced foil transparency. This symmetric configuration enables efficient Compton back-scattering and results in ultra-bright γ-photon emission with brightness of ~10²⁵ photons/s/mm²/mrad²/0.1%BW at 15 MeV and intensity of 5 × 10²³ Wcm^-2. Our first three-dimensional simulation with quantum-electrodynamics incorporated shows that a GeV positron beam with density of 2.5 × 10²² cm^-3 and flux of 1.6 × 10¹⁰/shot is achieved. Collective effects of the pair plasma may be also triggered, offering a window on investigating laboratory astrophysics at PW laser facilities.