A compact trampoline-in-the-middle system for acoustic frequency quantum optomechanics

Strained nanomechanical resonators can have extremely low mechanical loss at acoustic frequencies, spurring recent proposals for ultra-sensitive force detection and quantum experiments at room temperature. We present a quasi-monolithic membrane-in-the-middle system that incorporates a 10 ng Si₃N₄ trampoline resonator with a fundamental frequency of 40 kHz into a 100 um Fabry-Perot cavity with a finesse of 30,000. Prior to cavity assembly, we record a mechanical loss rate of 0.8 mHz (a quality factor of 50 million) for the fundamental trampoline mode, corresponding to a force sensitivity of 30 aN/√Hz and a zero-point displacement spectral density of 0.3 pm/√Hz. Mounting results in significant added loss; however, the large optomechanical cooperativity still reveals itself as Brownian motion exceeding the cavity linewidth by an order of magnitude. We present a technique to “load” the resonator into the cavity by radiation pressure feedback cooling. Eliminating mounting loss would result in a vacuum cooperativity of 10⁴, accessing a regime currently targeted with levitated nanoparticles.