Silicon optomechanical crystal resonator at millikelvin temperatures

Abstract

Optical measurements of a nanoscale silicon optomechanical crystal cavity with a mechanical resonance frequency of $3.6$ GHz are performed at subkelvin temperatures. We infer optical-absorption-induced heating and damping of the mechanical resonator from measurements of phonon occupancy and motional sideband asymmetry. At the lowest probe power and lowest fridge temperature (${T}_{\mathrm{f}}=10$ mK), the localized mechanical resonance is found to couple at a rate of ${\ensuremath{\gamma}}_{\mathrm{i}}/2\ensuremath{\pi}=400$ Hz (${Q}_{\mathrm{m}}=9\ifmmode\times\else\texttimes\fi{}{10}^{6}$) to a thermal bath of temperature ${T}_{\mathrm{b}}\ensuremath{\approx}270$ mK. These measurements indicate that silicon optomechanical crystals cooled to millikelvin temperatures should be suitable for a variety of experiments involving coherent coupling between photons and phonons at the single quanta level.