The vacuum field in a dielectric cavity where the medium has a rapidly
changing time-dependent refractive index exhibits nonclassical featur
es that are directly related to the distortion of the vacuum fluctuati
ons of the field. For appropriate parameters, these nonclassical effec
ts are appreciable enough that a suitable measure of the change in the
zero-point noise of the cavity field could provide a way to probe dis
tortions of the vacuum fluctuations.