Ultra-High-Q Membrane Optomechanics with a Twist

Abstract

The field of cavity optomechanics explores the interaction between optical and mechanicaldegrees of freedom mediated by radiation pressure. It first became relevant in the context of laser interferometric gravitational wave detection, where the optomechanical effects must be taken into account to improve the detection sensitivity. The continued excitement in this field is fueled by ultrasensitive optical detection of small forces, masses, and accelerations as well as prospects of observing quantum effects at macroscopic length scales. The canonical cavity optomechanical system consists of a Fabry-Perot cavity with a compliant end-mirror. The motion of the end mirror is coupled to the cavity field via radiation pressure. The concept that electromagnetic radiation exerts forces on material objects was first predicted by Maxwell and experimentally demonstrated by A. Ashkin and colleagues by trapping tiny particles using focused laser beams. Recent experiments in optomechanics have exploited radiation pressure forces to manipulate the center of mass motion of the nanomechanical resonator to cool the resonator to its motional ground state. Leveraging radiation pressure-based coupling, an optomechanical system can be employed as a tool for high-precision sensing applications ranging from atomic force microscopy to gravimetry. Central to the performance of optomechanical sensing is the high-quality factor (Q) nanomechanical resonator. Fabricating a high-Q resonator is akin to shielding it from its thermal environment. The goal is to maximize the number of coherent oscillations in the presence of thermal decoherence: a prerequisite for room temperature quantum experiments and reduced thermal force noise for ultrasensitive force detection. To achieve high Q, it is crucial to minimize mechanical dissipation in the system. Recently, mechanical resonators fabricated from strained silicon nitride (Si3N4) thin-films have demonstrated Q factors over 1 billion exploiting the phenomenon of dissipation dilution. Despite rapid advancements, a single platform that meets all these unique requirements ofa high-sensitivity integrated cavity optomechanical system is still an active area of research. In this thesis, I will discuss how a dielectric thin film can be realized as both a high-Q nanomechanical resonator and a high-reflectivity curved mirror by combining phononic and gradient photonic crystal patterning. I will explore its application in creating a vertically integrated on-chip optomechanical cavity. Moreover, I will address our unexpected discovery of high-Q torsional modes and how this finding enabled us to develop a state-of-the-art chip-scale gravimeter.