Digest of an Institution Monograph Artificial Dielectrics Having Refractive Indices Less than Unity

The term "artificial dielectric" is conveniently applied to any arrangement of conducting obstacles which affects the propagation of electromagnetic waves in a way similar to a solid dielectric. The essential property of a solid dielectric from the point of view of propagation is that the phase velocity within the dielectric is different from the free-space value. A quantitative indication of this difference is given by the refractive index, which is defined for any medium as the ratio of the phase velocity of a plane wave in free space to the phase velocity of the same wave in the medium. This definition can be extended to artificial dielectrics provided that the conducting obstacles are arranged in some regular manner. Within a solid dielectric the phase of a plane wave changes in a uniform manner from point to point, and it is easy to derive the appropriate value of the phase velocity. Near the conducting obstacles of an artificial dielectric there will be pronounced local variations in the electromagnetic fields, and before the phase velocity can be calculated these local variations must be averaged out. The value so obtained for the refractive index can then only be applied to volumes of space consisting of an exact multiple of the volume associated with each obstacle of the artificial dielectric. In practice this does not form a serious restriction, because the volume of the artificial dielectric used is usually very much larger than the volume occupied by each obstacle.