Enhanced coherent laser reflection from QED plasma creation

QED cascades can generate electron-positron pairs when the field exceeds the Schwinger limit. This condition is expected to become available with the rapid development of ultra-strong laser sources and particle accelerators. Recent studies [1-3] have shown that the formed pairs can exhibit collective plasma effects through, e.g., plasma-induced frequency upshifts in the laser spectrum. We will show that creation of QED plasma also induces strong coherent laser reflection at high reflection coefficient. The enhanced coherent laser reflection, and laser frequency upshift, serve as complementary signatures of QED plasma creation.