  A two-photon emission of a medium with periodically time-dependent refractive
index is considered. The emission results from the zero-point fluctuations of
the medium. Usually this emission is very weak. However, it can be strongly
enhanced if the resonant condition $\omega_0$ = 2.94 $c/l_0$ is fulfilled (here
$\omega_0$ and $l_0 $ are the frequency and the amplitude of the oscillations
of the optical length of the medium, respectively). Besides, a medium with
resonant oscillations of the optical length performs the phase conjugated
reflection with high efficiency. A similar resonant enhancement of the
two-quantum emission of other bosons is also predicted.
