To identify particles generated in a particle accelerator, physicists often determine their mass from measurements of the Cerenkov radiation cone produced by the particles; the Cerenkov radiation forms when the particle travels faster than the speed of light light in a transparent material (which must have a refractive index n of greater than 1, thus slowing light down).
The angle of the Cerenkov light cone provides the particle physicists a measure of the velocity of the particle, which helps them to identify the particle. The problem is that the light cone angle has a limit: all particles with high momentum (mass x velocity) generate light cones with the same angle. Hence, these particles are indistinguishable.
Now, Chalmers University of Technology (Göteborg, Sweden) researcher Philippe Tassin and his colleagues at the Free University of Brussels (Brussels, Belgium) have designed an optical metamaterial that manipulates the Cerenkov cone so that particles with high momentum get a distinct light cone angle too.1
"The result is that even particles with large momentum can be efficiently separated and identified," says Tassin.
The method relies on transformation optics, which are made possible by optical metamaterials. Carefully calculated variations of the metamaterial's refractive index causes the light to experience the material as being curved; therefore, it behaves differently from what we are used to. In the Chalmers scientists' material, Cerenkov radiation experiences the material as stretched in two different directions, giving rise to light cones with distinct angles.
"So far, transformation optics has mainly concerned with changing the light rays' paths through a material," says Tassin. "Now, we show that it is also possible to influence the generation of light."
1. Vincent Ginis et al., Physical Review Letters (2014); doi: http://dx.doi.org/10.1103/PhysRevLett.113.167402