Vertical dipole above a dielectric or metallic half-space - energy flow considerations
The emission pattern from a classical dipole located above and oriented perpendicular to a metallic or dielectric half space is calculated for a dipole driven at constant amplitude. This is a problem considered originally by Sommerfeld and analyzed subsequently by numerous authors. In contrast to most previous treatments, however, we focus on the energy flow in the metal or dielectric. It is shown that the radial Poynting vector in the metal points inwards when the frequency of the dipole is below the surface plasmon resonance frequency. In this case, energy actually flows of the interface at small radii. The Joule heating in the metal is also calculated and it is shown explicitly that Poynting's theorem holds for a cylindrical surface in the metal. When the metal is replaced by a dielectric having permittivity less than that of the medium in which the dipole is immersed, it is found that energy flows out of the interface for sufficiently large radii. In all cases it is assumed that the imaginary part of the permittivity of the metal or dielectric is much less than unity.