Abstract
The microscopic processes underlying electro-optic sampling of quantum-vacuum fluctuations are discussed, leading to the interpretation of these experiments in terms of an exchange of virtual photons. With this in mind it is shown how one can directly study the Purcell effect, i.e. the changes induced by cavities upon the quantum vacuum, in the frequency and time domains. This forges a link between electro-optic sampling of the quantum vacuum and geometry-induced vacuum effects.