Cavity optomechanics with Si3N4 membranes at cryogenic temperatures

We describe a cryogenic cavity-optomechanical system that combines Si3N4 membranes with a mechanically rigid Fabry?Perot cavity. The extremely high products of quality factor and frequency of the membranes allow us to cool a MHz mechanical mode to a phonon occupation of , starting at a bath temperature of 5?K. We show that even at cold temperatures thermally occupied mechanical modes of the cavity elements can be a limitation, and we discuss methods to reduce these effects sufficiently for achieving ground state cooling. This promising new platform should have versatile uses for hybrid devices and searches for radiation pressure shot noise.