Photonic metamaterials and transformation optics: A very brief introduction and review

Abstract

The major aim of optics & photonics is to obtain complete control on light propagation and light-matter interaction. In this context, materials play a crucial role. In materials, the propagation of light is influenced by the local refractive index n. The refractive index tells us by what factor the phase velocity of light inside the material is slower than the vacuum speed of light. Thus, one should actually rather call the refractive index the slowness factor of light. Microscopically, in usual materials at optical frequencies, the phase velocity is modified by electric dipoles (formed by the negatively charged electrons and the positive nuclei) that are excited by the electric component of the electromagnetic light wave. These electric dipoles re-radiate waves just like an antenna in radio engineering. The re-emitted wave excites further electric dipoles in the material that again re-radiate, etc. Thus, it is intuitively clear that, inside the material, light will propagate with a velocity different from that in free space. Usually, it is slower than in free space. Equivalently, the refractive index is larger than unity, i.e., n > 1. Under these conditions, the refractive index is the square root of the electric permittivity ε. n > 1 clearly implies that n > 0, which means that the phase-velocity vector and the vector of the electromagnetic energy flow, the Poynting vector, point into the same direction. Waves with this property are called forward waves. © 2013 Springer Science+Business Media Dordrecht.