Superradiant scattering from evanescent waves

Evanescent waves, such as the ones present at the interface between a laser and an overdense plasma, provide extremely localized fields that are able to induce rapid accelerations in charged particles. We show that such accelerations can generate high-energy, low-divergence, directional photon beams even for moderately relativistic electrons ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>γ</a:mi> <a:mo>≃</a:mo> <a:mn>10</a:mn> </a:mrow> </a:math> ). This process can also lead to superradiant emission of bright, ultrashort pulses of light, as long as the electron bunch is suitably modulated. Although never directly observed, this mechanism should leave clear signatures in laser-plasma acceleration experiments with minimal setup adaptations.