Nat. Commun. 8, 14901 (2017)

Cherenkov radiation is emitted by charged particles when they propagate in a dielectric medium at a speed that exceeds the phase velocity of light and is a common sight in nuclear physics. In conventional materials, the emission is in the same direction as the particle motion. Now, making use of a specially designed left-handed metamaterial (a material engineered to simultaneously possess a negative permeability and a negative permittivity), a research team from China and the US has observed reverse Cherenkov radiation that counterintuitively propagates in the opposite direction to that of the particles. The experiment took place in the microwave part of the electromagnetic spectrum (2–4 GHz). It involved a single sheet-electron-beam travelling through a metamaterial made from a square waveguide loaded with split-ring resonators and engineered to have both a negative permittivity and a negative permeability in the range 2.83–3.05 GHz.