On the equilibrium spectrum of surface polaritons thermally excited by solids

► Spectral energy densities of surface plasmon and phonon polaritons are calculated in case of thermodynamic equilibrium. ► Spectral distribution of surface polaritons in equilibrium is quite different from the Planckʼs law for photons in free space. ► Temperature dependence of the surface density of energy for two- and three-dimensional fluctuations is essentially different.