Complete reflection of nonlinear electromagnetic waves in underdense pair plasmas enabled by dynamically formed Bragg-like structures

In contrast to relativistically induced transparency in electron–ion plasmas, where nonlinear electromagnetic waves render initially opaque plasmas transparent, we show using kinetic simulations that such waves can instead make initially transparent pair plasmas fully reflective. The difference is mass symmetry, which eliminates charge-separation electric fields. As the wave compresses the pair plasma, weak reflection seeds density spikes that form a moving Bragg-like grating. Enhanced reflection enables a transition to a regime where the plasma–vacuum interface sustains complete reflection.