Silicon optomechanical crystal resonator at millikelvin temperatures

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-f = 10 mK), the localized mechanical resonance is found to couple at a rate of gamma(i)/2 pi = 400 Hz (Q(m) = 9x10(6)) to a thermal bath of temperature Tb approximate to 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.