Following Purcell, the rate of spontaneous emission of atoms and molecules strongly depends on the emitters’ environment and the local density of photonic states. Numerous known examples of environments with engineering density of photonic states include cavities and photonic band crystals.

The research in hyperbolic metamaterials (in which dielectric permittivities in orthogonal directions have opposite signs), originally stimulated by tantalizing possibilities offered by the absence of diffraction limit in a hyperlens, has uncovered a number of novel effects resulting from a broadband singular behavior of the density of photonic states in these materials.