  A theoretical consideration of the so-called "membrane-in-the-middle"
optomechanical cavity revealed that it undergoes a spontaneous symmetry
breaking as a function of the transparency of the membrane. Such typical
features of this phenomenon as a square-root development of the order parameter
and divergency of the critical susceptibility were identified. In the contract
to classical spontaneous-symmetry-breaking systems of ferromagnets and
ferroelectrics, in the considered system, this divergency remains, due to
interference effects, an "internal" property of the system, which does not
reveal itself in any divergency of its observables. A spontaneous symmetry
breaking in an optomechanical cavity might pave a new way to enhanced
optomechanical interactions.
