Opto-Acousto-Mechanical Sensing

We explore what happens if we replace the electromagnetic waves in a cavity optomechanical system with acoustic waves. In particular, we will discuss the design and fabrication of a cavity acousto-mechanical system consisting of a centimeter-scale Si tuning fork as a mechanical resonator, with a high-stress, high-Q silicon nitride string suspended between the tuning fork tines acting as a higher-frequency acoustic wave resonator. These two resonators are parametrically coupled, with a hierarchy of frequency scales that is completely analogous to cavity optomechanics in the resolved sideband regime. The motion of the string is detected with an optical interferometer, and we can excite the string with a piezoelectric shaker, allowing us to both feedback cool the string and pump it with a large coherent drive. We demonstrate many of the fundamental effects familiar in cavity optomechanics – acoustic cooling of the tuning fork mode, strong coupling between the string and tuning fork, and high sensitivity readout of the tuning fork motion via detection of the string. We assess the suitability of this system for applications such as precision accelerometry and gravimetry, and for operation in the quantum regime and searches for new physics with macroscopic mechanical systems.