  Cooling mechanical resonators is of great importance for both fundamental
study and applied science. We investigate the hybrid optomechanical cooling
with a three-level atomic ensemble fixed in a strong excited optical cavity. By
using the quantum noise approach, we find the upper bound of the noise spectrum
and further present three optimal parameter conditions, which can yield a small
heating coefficient, a large cooling coefficient, and thus a small final phonon
number. Moreover, through the covariance matrix approach, results of numerical
simulation are obtained, which are consistent with the theoretical
expectations. It is demonstrated that our scheme can achieve ground state
cooling in the highly unresolved sideband regime, within the current
experimental technologies. Compared with the previous cooling methods, in our
scheme, there are fewer constraints on the drive strength of atomic ensemble
and number of atoms in the ensemble. In addition, the tolerable ranges of
parameters for ground state cooling are extended. As a result, our scheme is
very suitable for experiments and can be a guideline for the research of hybrid
optomechanical cooling.
