  Cooling and Entanglement in optomechanical systems coupled through radiation
pressure and photothermal force is studied. To develop the photothermal model,
we derive an expression for deformation constant of the force. Exploiting
linearized quantum Langevin equations we investigate dynamics of such systems.
According to our analysis, in addition to separate action of radiation pressure
and photothermal force, their cross correlation effect plays an important role
in dynamics of the system. We also achieve an exact relation for the phonon
number of the mechanical resonator in such systems, and then we derive an
analytical expression for it at weak coupling limit. At strong coupling regime,
we show that utilizing the photothermal pressure makes the ground state cooling
more approachable. The effect of photothermal force on the optomechanical
entanglement is investigated in detail. According to our exact numerical and
approximate analytical studies, even though the photothermal force is naturally
a dissipative force, it can improve the optomechanical entanglement both
quantitatively and qualitatively.
