Thermodynamic functions of fluctuating electromagnetic fields within a heterogeneous system

Abstract

Thermodynamic functions of the equilibrium fluctuating electromagnetic fields within a nonuniform dispersive and absorptive system are calculated and discussed. The near-interface subsystem of the fields deserves special attention. Density of states and spectral characteristics of thermal surface waves in equilibrium at the interface between two media, one of them being lossy, are calculated for the first time. Thermodynamic characteristics of thermal fields are calculated both in vacuum and inside bulk lossy material described by spatially local and nonlocal optical responses. It was shown that the higher the temperature of the system, the smaller the relative contribution of the evanescent part of thermodynamic functions of thermal fields in vacuum compared with those of the black-body radiation. It is suggested that the evanescent part of the thermally excited fields can be treated as the physical transition layer localized near the interface between two subsystems of photons in vacuum and photons in bulk matter. In fact, such a layer forms a physical boundary between those subsystems of thermal photons in equilibrium.