Dissertation Defense: Mitchell Underwood, Yale University, “Cryogenic Optomechanics with a Silicon Nitride Membrane”

The field of optomechanics involves the study of the interaction between light and matter via the radiation pressure force. Though the radiation pressure force is quite weak compared with forces we normally experience in the macroscopic world, modern optical and microwave resonators are able to enhance the radiation pressure force so that it can be used to both measure and control the motion of macroscopic mechanical oscillators. Recently, optomechanical systems have reached a regime where the sensitivity to mechanical motion is limited only by quantum effects. Together with optical cooling techniques such as sideband cooling, this sensitivity has allowed experiments to probe the quantum behaviors of macroscopic objects, and also the quantum limits of measurement itself. In this dissertation I describe the physics underlying the modern field of optomechanics and provide an overview of experimental accomplishments of the field such as ground state cooling of mechanical oscillators, detection of radiation pressure shot noise, and preparation, storage, and transfer of quantum states between macroscopic objects and the electromagnetic field. I then describe the specific experimental work done in pursuit of my degree involving the ground state cooling of a silicon nitride membrane in a high finesse Fabry-Perot cavity, and a systematic characterization of the dynamics that occur when the membrane is coupled to two nearly degenerate cavity modes at an avoided crossing in the cavity spectrum. In the section on ground state cooling, particular attention is given to the influence of classical laser noise on the measurement of the membrane’s motion at low phonon occupancies, and techniques for laser noise measurement and reduction are discussed.