  We develop a general theory of the quantum vacuum radiation generated by an
arbitrary time-modulation of the vacuum Rabi frequency for an intersubband
transition of a doped quantum well system embedded in a semiconductor planar
microcavity. Both non-radiative and radiative losses are included within an
input-output quantum Langevin framework. The intensity and the main spectral
signatures of the extra-cavity emission are characterized as a function of the
amplitude and the frequency of the vacuum modulation. A significant amount of
photon pairs, which can largely exceed the black-body radiation in the mid and
far infrared, is shown to be produced with realistic parameters. For a large
amplitude resonant modulation, a parametric oscillation regime can be also
achieved.
