A gain route to reversed Cherenkov radiation.

In his landmark paper from 1968, Veselago showed that Cherenkov radiation can be reversed in materials with a negative index of refraction, inspiring substantial research into such materials in what became one of the cornerstone concepts of the field of metamaterials. Following and ongoing investigations of reversed Cherenkov radiation considered it impossible to occur from a homogeneous isotropic slab with a positive refractive index. Here, we break this long-held belief by finding the emergence of reversed Cherenkov radiation from a positive-index isotropic slab by exploiting optical gain. We find precise conditions under which the backward-propagating Cherenkov radiation is maintained while the forward-propagating one is suppressed. Counterintuitively, the intensity and the angular spread of reversed Cherenkov radiation can be made robust to the slab thickness. Under this scenario, increasing the optical gain could decrease the intensity and increase the radiation angular spread.