Optical cooling of a 122 kHz mechanical resonator

We demonstrate radiation pressure cooling of a 122-kHz, 60-ng mechanical resonator in an optical cavity. We use a dilution refrigerator to achieve a low base temperature ($<$ 100 mK). The resonators consist of high reflectivity mirrors suspended on a stressed silicon nitride cross resonator. Due to their low frequency, high Q ($>$40000) and high finesse ($>$ 10000) these devices are excellent candidates for demonstrating quantum behavior of macroscopic systems, with the possibility of achieving quantum superpositions, entanglement with external degrees of freedom, and studying exotic decoherence mechanisms.