Generalised superradiance with less than a particle per wavelength

Superradiance, a classical concept first introduced by Robert Dicke in 1954, is the anomalous radiance describing coherent photon emission from a collection of light-emitting particles. Superradiant regimes lead to peak radiation intensities that scale with the number of particles squared. As a result, superradiance is at the core of the most advanced light sources, such as free electron lasers. This effect is intuitively expected when the distance between light-emitting particles is much smaller than the photon wavelength. This was the superradiance condition originally proposed in the seminal work by Dicke, and corresponds to the operation regime of free electron lasers. Here, we predict a new superradiance effect that holds even in the limit of vanishing number of particles per radiation wavelength. At the heart of this superradiant emission concept is a relativistic particle bunch with specific spatiotemporal modulations. We discuss how these bunches could be produced in compact plasma accelerators driven by structured lasers with orbital angular momentum. Furthermore, we also show that the required spatiotemporal modulations could also appear self-consistently during the interaction of a relativistic particle bunch with an intense laser. We thus show that the latter scenario leads to a previously unrecognized superradiant Thomson scattering regime. We demonstrate our findings using theory and simulations using RaDiO (Radiation Diagnostic for Osiris). Instead of focusing on the spectral features of radiation emission, RaDiO captures the spatiotemporal features of radiation emission, being ideal to exemplify key features of the new superradiant emission concept.