  The dynamical Casimir effect (DCE) is the production of photons by the
amplification of vacuum fluctuations. In this paper we demonstrate new
resonance conditions in DCE that potentially allow the production of optical
photons when the mechanical frequency is smaller than the lowest frequency of
the cavity field. We consider a cavity with one mirror fixed and the other
allowed to oscillate. In order to identify the region where production of
photons takes place, we do a linear stability analysis and investigate the
dynamic stability of the system under small fluctuations. By using a numerical
solution of the Heisenberg equations of motion, the time evolution of the
number of photons produced in the unstable region is studied.
