  Optomechanical systems have been attracting intensive attention in various
physical experiments. With an optomechanical system, the displacement of or the
force acting on a mechanical oscillator can be precisely measured by utilizing
optical interferometry. As a mechanical oscillator, a suspended mirror is often
used in over milligram scale optomechanical systems. However, the tiny
suspended mirror in a linear cavity can be unstable in its yaw rotational
degree of freedom due to optical radiation pressure. This instability curbs the
optical power that the cavity can accumulate in it, and imposes a limitation on
the sensitivity. Here, we show that the optical radiation pressure can be used
to trap the rotational motion of the suspended mirror without additional active
feedback control when the $g$ factors of the cavity are negative and one mirror
is much heavier than the other one. Furthermore, we demonstrate experimentally
the validity of the trapping. We measured the rotational stiffness of a
suspended tiny mirror with various intracavity power. The result indicates that
the radiation pressure of the laser beam inside the cavity actually works as a
positive restoring torque. Moreover, we discuss the feasibility of observing
quantum radiation pressure fluctuation with our experimental setup as an
application of our trapping configuration.
