In 1968, Veselago  predicted that there could still be propagating waves in a medium that had simultaneously negative permittivity ε and permeability μ, because the product of ε and μ would be positive. However, to ensure energy conservation, he concluded that the refractive index must use the negative square root of the product of ε and μ (i.e., n = ). A consequence is certain unusual optical features in negative-index media. The electric field vector E, magnetic field vector H and wave vector k are a left-handed triplet, the basis for calling materials with simultaneously negative ε and μ “left-handed materials” (LHMs). LHMs would have novel optical properties. Light at non-normal incidence would bend to the side opposite that in a normal RHM; positive lenses would become negative; a flat slab could focus. The phase velocity of an electromagnetic wave would be opposite to the direction of energy flux, resulting in a reversed Doppler effect. Photons would have negative momentum and apply tension to the interface upon reflection. Recently, this kind of material has been demonstrated experimentally to exist. Shelby et al.  measured the scattering angle of the transmitted beam through a prism manufactured from a composite material consisting of a two-dimensional array of copper wires and split ring resonators and showed that the effective refractive index of the material is negative at microwave frequencies. Recent theoretical studies also showed that some photonic crystals might have negative-refraction properties in the near infrared spectral region .
Radiative Properties of Multilayer Thin Films With Positive and Negative Refractive Indexes
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Fu, C, Zhang, ZM, & Tanner, DB. "Radiative Properties of Multilayer Thin Films With Positive and Negative Refractive Indexes." Proceedings of the ASME 2002 International Mechanical Engineering Congress and Exposition. Heat Transfer, Volume 2. New Orleans, Louisiana, USA. November 17–22, 2002. pp. 191-192. ASME. https://doi.org/10.1115/IMECE2002-32771
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