Librational Feedback Cooling

TL;DR

This paper demonstrates a method to cool librational motion of optically trapped silica microspheres by phase modulating an external electric field, advancing control over microscopic mechanical systems.

Contribution

It introduces a novel phase modulation technique to cool librational degrees of freedom in optically trapped microspheres, enabling better control of their mechanical motion.

Findings

Achieved cooling of librational motion in silica microspheres.

Quantified cooling by damping time and mode temperature.

Provides insights into charge mobility at microscopic scales.

Abstract

Librational motion, whereby a rigid body undergoes angular oscillation around a preferred direction, can be observed in optically trapped, silica microspheres. We demonstrate the cooling of one librational degree of freedom for $\sim 5~\mu$m diameter spheres that have been induced to rotate with an external electric field coupled to their electric dipole moment. Cooling is accomplished by adding a phase modulation to the rotating field. The degree of cooling is quantified by applying a $\pi/2$ shift to the phase of the electric field and fitting the resulting exponential decay of the librational motion to obtain a damping time, as well as estimating a mode temperature from the observed libration in equilibrium. The result is an important step in the study of the dynamics of trapped microspheres, crucial to cooling the mechanical motion to its ground state, as well as providing insights regarding the charge mobility in the material at microscopic scales.