Nonlinear optomechanics in multimode nanoscale resonators

In this thesis, we theoretically and experimentally investigate the enhancement of nonlinear optomechanical coupling in multimode systems. We study the membrane-in-the-middle system, where we learn that it resonantly enhances the intrinsic nonlinearity, and we find that the enhancement is limited by the degree of sideband resolution. More generally, we analyse what systems are most promising to produce significant nonlinear effects at the quantum level and describe new insights from perturbation theory. We design, fabricate and test an experimental system equivalent to the membrane-in-the-middle in optomechanical crystal nanobeams, and we use active thermal tuning for frequency-matching of the two cavities. We find multiple high quality-factor optical and mechanical modes with large optomechanical coupling rates. We use this system to demonstrate enhanced linear and nonlinear optomechanical transduction. Additionally, we study mechanical pulse propagation in phononic crystal waveguides and simulate possible designs for a beam splitter in such systems.