Fig. 4: 3D PIC simulations of e⁻/e⁺ beam colliding with a laser pulse.

a radiation spectrum, b microbunch train, and c particle distribution produced from a beam of peak density 10²⁰ particle/cm³ colliding with a FWHM_L = 100 fs laser pulse. This is repeated in (d–f) with a lower density beam 10¹⁸ particle/cm³ colliding with a longer pulse FWHM_L = 400 fs. The spectra are observed along the  + z direction and the microbunches are pictured shortly after the laser pulse peak. Coherent emission occurs due to the formation of nanoscale microbunches separated by approximately λ₁ ≈ 55 nm. When the simulations are repeated with an electron beam, no microbunching or coherent emission occurs. Note that “a.u.” denotes arbitrary units.