Silicon-nitride nanosensors toward room temperature quantum optomechanics

Observation of quantum phenomena in cryogenic, optically cooled mechanical resonators has been recently achieved by a few experiments based on cavity optomechanics. A well-established experimental platform is based on a thin film stoichiometric ($ Si_3 N_4 $) nanomembrane embedded in a Fabry-Perot cavity, where the coupling with the light field is provided by the radiation pressure of the light impinging on the membrane surface. Two crucial parameters have to be optimized to ensure that these systems work at the quantum level: the cooperativity $ C$ describing the optomechanical coupling and the product $ Q \times
u$ (quality factor - resonance frequency) related to the decoherence rate. A significant increase of the latter can be obtained with high aspect-ratio membrane resonators where uniform stress dilutes the mechanical dissipation. Furthermore, ultra-high $Q \times
u$ can be reached by drastically reducing the edge dissipation via clamp-tapering and/or by soft-clamping, virtually a clamp-free resonator configuration. In this work, we investigate, theoretically and experimentally, the edge loss mechanisms comparing two state-of-the-art resonators built by standard micro/nanofabrication techniques. The corresponding results would provide meaningful guidelines for designing new ultra-coherent resonating devices.