Influence of external boundaries on the Casimir effect between metallic plates

We consider the Casimir effect between two identical (free-electron) metallic plates with emphasis on the influence of their external boundaries on it. We analyze the contributions of different types of the system eigenmodes to the Casimir energy and force paying particular attention to the role of surface plasmon polariton (SP) modes, which are very sensitive to the dielectric properties at the boundaries. Since their contribution to the Casimir energy and force can be easily calculated separately, it is important to determine the conditions under which the contribution of SP modes is sufficient for the interpretation of the Casimir effect. We perform such calculations for a range of system parameters and conclude that, in all cases, the small separation between the metallic plates is a precondition that SP modes have a dominant influence on the Casimir effect. This precondition is sufficient for thick plates, but in the case of thin plates the contribution of SP modes vs that of photonic modes depends significantly on the dielectric (reflection) properties of the medium supporting the plates.