  Ground state cooling of mechanical resonator is a way to generate macroscopic
quantum states. Here we present a study of optomechanical cooling under the
drive of square pulses without smooth profile. By illustrating the dynamical
processes of cooling, we show how to choose the amplitudes and durations of
square pulses, as well as the intervals between them, so that a mechanical
resonator can be quickly cooled down to its ground state. Compared with the
cooling under a continuous-wave drive field, the ground state cooling of a
mechanical resonator can be performed more efficiently and flexibly by using
square pulse drives. At certain times of such cooling process, the thermal
phonon number under square pulse drives can become even lower than the
theoretical limit for the cooling with a continuous-wave drive field of the
same amplitude.
