  We review the description and modeling of transport phenomena among the
electron systems coupled via scalar or vector photons. It consists of three
parts. The first part is about scalar photons, i.e., Coulomb interactions. The
second part is with transverse photons described by vector potentials. The
third part is on $\phi=0$ or temporal gauge, which is a full theory of the
electrodynamics. We use the nonequilibrium Green's function (NEGF) formalism as
a basic tool to study steady-state transport. Although with local equilibrium
it is equivalent to the fluctuational electrodynamics (FE), the advantage of
NEGF is that it can go beyond FE due to its generality. We have given a few
examples in the review, such as transfer of heat between graphene sheets driven
by potential bias, emission of light by a double quantum dot, and emission of
energy, momentum, and angular momentum from a graphene nanoribbon. All of these
calculations are based on a generalization of the Meir-Wingreen formula
commonly used in electronic transport in mesoscopic systems, with materials
properties represented by photon self-energy, coupled with the Keldysh equation
and the solution to the Dyson equation.
