Sandwich in the middle: enhancing the optomechanical coupling

Multi–element systems of micro/nano–mechanical resonators offer promising prospects for enhanced optomechanical performances [1,2], and for the exploration of multi–oscillators synchronization [3]. A solution, capable of providing systems with ratio between the single–photon optomechanical coupling rate and the cavity decay rate enhanced by orders of magnitude, exploits quantum interference in multi–element optomechanical setups. Although the simplest two–membrane sandwich in an optical cavity is a paradigm for the realization of strong–coupling optomechanics, and the observation of collective mechanical effects (such as synchronization), no experimental studies of these phenomena have been reported till now. We report on the first experimental characterization of the optical, mechanical, and especially optomechanical properties of a sandwich constituted of two parallel membranes within an optical cavity [4]. The membrane–sandwich we use in our experiment is constituted of two low–stress Si3N4 square membranes, with a side of 1 mm, and a thickness of 100 nm. One of the membranes is glued on a piezo, for scanning the membrane–cavity length. The whole membrane–sandwich mount is attached to another piezo to displace in a controlled way the center of mass of the two membranes inside the Fabry–Perot cavity. To estimate the optomechanical coupling strength achievable with our system, we inserted the membrane–sandwich in an optical cavity 90 mm–long, driven with a 1064 nm laser beam