Dispersion forces inside metallic waveguides

We consider the dispersion energy of a pair of dipoles embedded in a metallic waveguide with transverse dimension a smaller than the characteristic dipolar wavelength. We find that a sets the scale that separates retarded, Casimir-Polder-like from quasistatic, van der Waals-like interactions. Whereas in the retarded regime the energy decays exponentially with interdipolar distance, typical of evanescent waves, in the van der Waals regime, the known free-space result is obtained. This short-range scaling implies that the additivity of the dispersion interactions inside a waveguide extends to denser media, along with modifications to related Casimir effects in such structures.