Double-negative metallic materials
Metamaterials: from microwaves to optics

The first approach to exploring these negative refraction phenomena is to define the index based on the permittivity ε and permeability µ of a homogeneous medium. Strictly speaking, n ² = εµ. The intuitive solution is to consider the positive root of n to characterize a propagative medium. Nevertheless, in theory, Veselago showed that, provided ε and µ are both negative, the negative root of n ² leads to propagative solutions that in no way contradict Maxwell's equations... but, on the contrary, generalize classical laws based on the refractive index (Snell-Descartes law, Doppler effect, Cerenkov effect...). Although the existence of a negative-permittivity medium was accepted as early as the late 1960s, the idea of a negative-permeability medium, or even a medium combining the two, did not seem practically...