Flipping a photonic shock wave

When charged particles such as electrons travel through a dielectric medium with a speed greater than the phase velocity of the light in the medium, electromagnetic radiation is emitted that falls into a cone fanning out in the forward direction [Fig. 1, top left]. This phenomenon is called Čerenkov radiation, named after the Russian scientist who first characterized it rigorously and was awarded the Nobel Prize in Physics in 1958. Most people are familiar with Čerenkov radiation from the blue glow of an underwater nuclear reactor as it emits energetic charged particles. The angle of the Čerenkov emission cone is related in a simple way to the particle velocity. This unique feature enables a wide range of applications, from the measurement of fast particles in high-energy physics, the characterization of fission rate in nuclear reactors, to the detection of labeled biomolecules. Now in a paper appearing in Physical Review Letters, Sheng Xi and colleagues at Zhejiang University, China, and the Massachusetts Institute of Technology, US, experimentally demonstrate that the direction of the cone of Čerenkov radiation can be reversed in artificially engineered composite media, namely, metamaterials [1].