  We study the generation of coherent phonons in a superconductor by ultrafast
optical pump pulses. The nonequilibrium dynamics of the coupled Bogoliubov
quasiparticle-phonon system after excitation with the pump pulse is analyzed by
means of the density-matrix formalism with the phonons treated at a full
quantum kinetic level. For ultrashort excitation pulses, the superconductor
exhibits a nonadiabatic behavior in which the superconducting order parameter
oscillates. We find that in this nonadiabatic regime the generation of coherent
phonons is resonantly enhanced when the frequency of the order-parameter
oscillation is tuned to the phonon energy, a condition that can be achieved in
experiments by varying the integrated pump pulse intensity.
