Doppler cooling a microsphere

TL;DR

This paper proposes a method to cool the motion of a microsphere using Doppler cooling via whispering gallery mode resonances, achieving microKelvin temperature limits.

Contribution

It introduces a novel Doppler cooling technique for microspheres leveraging WGM resonances, extending atomic cooling concepts to macroscopic particles.

Findings

Cooling times of seconds for a 20 micron silica microsphere

Doppler temperature limit in the microKelvin regime

Potential for controlled damping of microsphere motion

Abstract

Doppler cooling the center-of-mass motion of an optically levitated microsphere via the velocity dependent scattering force from narrow whispering gallery mode (WGM) resonances is described. Light that is red detuned from the WGM resonance can be used to damp the center-of-mass motion in a process analogous to the Doppler cooling of atoms. Leakage of photons out of the microsphere when the incident field is near resonant with the narrow WGM resonance acts to damp the motion of the sphere. The scattering force is not limited by saturation, but can be controlled by the incident power. Cooling times on the order of seconds are calculated for a 20 micron diameter silica microsphere trapped within optical tweezers, with a Doppler temperature limit in the microKelvin regime.