Nature Physics ( IF 19.256 ) Pub Date : 2020-09-14 , DOI: 10.1038/s41567-020-0995-5 J. Vieira; M. Pardal; J. T. Mendonça; R. A. Fonseca
Superradiance is the anomalous radiance describing coherent photon emission from a gas. It plays a crucial role in atomic physics, quantum mechanics and astrophysics. Because the intensity of superradiant light beams is proportional to the number of particles squared, superradiance is also at the core of today’s most powerful light sources. Superradiant emission is intuitively expected when the distance between light-emitting particles is much smaller than the photon wavelength. Here, we break this assumption by predicting a never considered superradiance effect that holds even when the particle number per wavelength vanishes. We discover that a bunch of relativistic charged particles arranged in certain ways can generate an optical shock along the Vavilov–Cherenkov angle in vacuum, thereby concentrating broadband radiation in any spectral region into single light bullets. The process leaves clear experimental signatures, and we illustrate it in the form of a previously unrecognized nonlinear superradiant Thomson scattering. This concept may enable new forms of superradiant emission in advanced light sources, atomic physics systems, and unlock coherent emission in plasma accelerators.