  The spectral functions are studied in conjunction with the dyadic Green's
functions for various media. The dyadic Green's functions are found using the
eigenfunction expansion method for homogeneous, inhomogeneous, periodic,
lossless, lossy, and anisotropic media, guided by the Bloch- Floquet theorem.
For the lossless media cases, the spectral functions can be directly related to
the photon local density of states, and hence, to the electromagnetic energy
density. For the lossy case, the spectral function can be related to the field
correlation function. Because of these properties, one can derive properties
for field correlations and the Langevin-source correlations without resorting
to the fluctuation dissipation theorem. The results are corroborated by the
fluctuation dissipation theorem. An expression for the local density of states
for lossy, inhomogeneous, and dispersive media has also been suggested.
