  We consider the nonstationary circuit QED setup in which a 3-level artificial
atom in the $\Delta$-configuration interacts with a single-mode cavity field of
natural frequency $\omega $. It is demonstrated that when some atomic energy
level(s) undergoes a weak harmonic modulation, photons can be generated from
vacuum via effective 1- and 3-photon transitions, while the atom remains
approximately in the ground state. These phenomena occur in the dispersive
regime when the modulation frequency is accurately tuned near $\omega $ and
$3\omega $, respectively, and the generated field states exhibit strikingly
different statistics from the squeezed vacuum state attained in standard cavity
dynamical Casimir effect.
