Engineering the phononic density of states in a flip-chip electro-optomechanical system

Many optomechanical systems rely on mechanical clamping to support suspended structures. However, connecting these structures to a bulk substrate can add loss and introduce a noisy background of modes. A common technique to improve the coherence of mechanical modes is to pattern the substrate, which reduces the phononic density of states. We study the performance of phononic patterning in a flip-chip architecture that uses posts to define the spacing between the chips. We find that the mechanical quality factor and the presence of unwanted substrate modes is sensitive to the placement of these posts. These posts make it possible to reliably place a membrane optomechanical system about 200 nanometers above an electrical circuit.