  Electro-optical sampling has been recently used to perform
spectrally-resolved measurements of electromagnetic vacuum fluctuations. In
order to understand which information on the ground state of an interacting
system can be acquired thanks to this technique, in this paper we will develop
the quantum theory of electro-optical sampling in arbitrary dispersive
dielectrics. Our theory shows that a measure of the time correlations of the
vacuum fluctuations effectively implements an ellipsometry measurement on the
quantum vacuum, allowing to access the frequency-dependent dielectric function.
We discuss consequences of these results on the possibility to use
electro-optical sampling to probe the population of ground-state virtual
photons in the ultrastrong light-matter coupling regime.
