Angular trapping of a linear-cavity mirror with an optical torsional spring

TL;DR

This paper demonstrates that optical radiation pressure can passively trap the rotational motion of a suspended mirror in a linear cavity, enhancing stability and sensitivity in optomechanical systems, with experimental validation and potential quantum applications.

Contribution

The study introduces a passive optical trapping method for a suspended mirror's rotation using radiation pressure, eliminating the need for active feedback control.

Findings

Radiation pressure can stabilize the mirror’s rotational degree of freedom.

Experimental measurements confirm the trapping effect and positive restoring torque.

Feasibility of observing quantum radiation pressure fluctuations is discussed.

Abstract

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.