  Multiphonon state plays an important role in quantum information processing
and quantum metrology. Here we propose a scheme to realize dynamical emission
of phonon pairs based on the technique of stimulated Raman adiabatic passage in
a single cavity optomechanical system, where the optical cavity is driven by
two Gaussian pulse lasers. By exploring quantum trajectories of the state
populations and the average phonon number, we find that the dynamical
phonon-pair emission can be realized under the appropriate parameter conditions
and is tunable by controlling the time interval between the consecutive pulses
of pump lasers. In particular, the numerical results for the standard and
generalized second-order correlation functions of the mechanical mode show that
the system can behave as an antibunched phonon-pair emitter. Our proposal can
be extended to achieve an antibunched $n$-phonon emitter, which has potential
applications for on-chip quantum communications.
