General theory of electromagnetic fluctuations near a homogeneous surface in terms of its reflection amplitudes

We derive general expressions for the fluctuating electromagnetic field outside a homogeneous material surface. The analysis is based on general results from the thermodynamics of irreversible processes, and requires no consideration of the material interior, as it only uses knowledge of the reflection amplitudes for its surface. Therefore, our results are valid for all homogeneous surfaces, including layered systems and metamaterials, at all temperatures. In particular, we obtain formulas for the near-field region, which are important for interpreting the numerous current experiments probing proximity effects for macroscopic and/or microscopic bodies separated by small empty gaps. By use of Onsager’s reciprocity relations, we obtain also the general symmetry properties that must be satisfied by the reflection matrix of any material.