Simultaneous cooling of coupled mechanical oscillators using whispering gallery mode resonances

TL;DR

This paper demonstrates the simultaneous cooling of two coupled mechanical oscillators, a microsphere-cantilever and a tapered optical fiber, using whispering gallery mode resonances for enhanced feedback control, achieving mode temperatures below 10 K.

Contribution

It introduces a novel method of using whispering gallery mode resonances for concurrent cooling of coupled micro-mechanical oscillators, including both the resonator and its waveguide.

Findings

Achieved mode temperatures below 10 K for the dominant mode.

Demonstrated stabilization at the picometer level.

First use of WGM resonances to cool both the resonator and waveguide modes.

Abstract

We demonstrate simultaneous center-of-mass cooling of two coupled oscillators, consisting of a microsphere-cantilever and a tapered optical fiber. Excitation of a whispering gallery mode (WGM) of the microsphere, via the evanescent field of the taper, provides a transduction signal that continuously monitors the relative motion between these two microgram objects with a sensitivity of 3 pm. The cavity enhanced optical dipole force is used to provide feedback damping on the motion of the micron-diameter taper, whereas a piezo stack is used to damp the motion of the much larger (up to $180\,\mu$m in diameter), heavier (up to $1.5\times 10^{-7}\,$kg) and stiffer microsphere-cantilever. In each feedback scheme multiple mechanical modes of each oscillator can be cooled, and mode temperatures below 10 K are reached for the dominant mode, consistent with limits determined by the measurement noise of our system. This represents stabilization on the picometer level and is the first demonstration of using WGM resonances to cool the mechanical modes of both the WGM resonator and its coupling waveguide.