  Recent progress in electro-optic sampling has allowed direct access to the
fluctuations of the electromagnetic ground state. Here, we present a
theoretical formalism that allows for an in-depth characterisation and
interpretation of such quantum-vacuum detection experiments by relating their
output statistics to the quantum statistics of the electromagnetic vacuum
probed. In particular, we include the effects of absorption, dispersion and
reflections from general environments. Our results agree with available
experimental data while leading to significant corrections to previous
theoretical predictions and generalises them to new parameter regimes. Our
formalism opens the door for a detailed experimental analysis of the different
characteristics of the polaritonic ground state, e.g. we show that transverse
(free-field) as well as longitudinal (matter or near-field) fluctuations can be
accessed individually by tuning the experimental parameters.
