A short and largely traditional review of the concept of group velocity, prepared for the forthcoming Encyclopedia of Nonlinear Science Waves often propagate as a packet, or group, within which there are several wave crests. A common illustration is given by the wave pattern formed when a stone is thrown into a pond. An axially symmetric ring of waves on the water surface propagates outwards as...

In more detail, consider a classical model of matter in which spectral lines are associated with oscillators. In particular, consider a gas of atoms with two closely spaced spectral lines of angular frequencies ω1,2 = ω0 ± Δ/2, where Δ ω0. Each line has the same damping constant (and spectral width) γ. Ordinarily, the gas would exhibit strong absorption of light in the vicinity of the spectral ...

The Bruggeman formalism is implemented to estimate the refractive index of an isotropic, dielectric, homogenized composite medium (HCM). Invoking the well–known Hashin– Shtrikman bounds, we demonstrate that the group velocity in certain HCMs can exceed the group velocities in their component materials. Such HCMs should therefore be considered as metamaterials. PACS numbers: 41.20.Jb, 42.25.Dd, ...