Phononics: Engineering and control of acoustic fields on a chip

Abstract

Micro and nanomechanical systems play an important role in modern science and technology. They are indispensable for precision sensing, navigation and communication. Over the past decade, the rapid advances in nano-fabrication and measurement science have enabled quantum control of mechanical devices by integrating them to optical and microwave cavities, in the growing field of quantum optomechanics. However, experiments in quantum optomechanics at room temperature still face significant challenges. Perhaps the most demanding condition to perform experiments of this nature is reducing noise level due to coupling of the device to its environment through mechanical vibrations, phonons. In this thesis, we engineer micromechanical devices that confine mechanical excitations, decoupling them from their environment. The engineered design of these resonators combines a built-in suspended phononic low pass filter with a trampoline design made of top quality SiC single crystal. Results with quality factors Q~4x10^8 show the efficiency of these resonators. This is the largest Q for a system of its kind with such a large mesoscopic mode size ~0.5 mm^2 and resonance frequency f~220 kHz. The ultra-high Q mechanical resonators we developed can be used for quantum optomechanics experiments at room temperature. Similar to electrons, phonons propagate through material and are characterized by their dispersion relation. By engineering the properties of the material it is possible to confine and guide phonons through phononic channels. The importance of guided phonons relies on the fact that guided signals are the back-bone of all communication systems. The existing platforms for mechanical channels rely on the inclusion of phononic crystals for phonon confinement. However, phononic crystals base their functionality on acoustic interference, limiting its scalability. In this thesis, we designed, fabricated and characterized the basic components for a phononic circuitry platform based on highly stressed silicon nitride membranes on Si. These phononic waveguides share a similar mathematical framework with to photonic waveguides. Our phononic waveguides are single mode for a range of frequencies. In this region, the guided mode experiences low dissipation. We also show that there is a cut-off frequency at which the excitations cannot propagate, completely analogous to the photonic case. This phononic ``wires'' could in principle be used as the fundamental element for mechanics based communication networks. In the last chapter of this thesis, we propose a magnetomechanical system, where the mechanical system couples through the momentum to an electromagnetic field. By coupling the momentum to an electromagnetic field, it is possible to perform non-demolition measurement protocols that allow us to measure directly the position of the oscillator. By enhancing the coupling between the mechanics and the electromagnetic field we predict that the ground state of the two systems get entangled. We designed a system that can achieve coupling rates as large as a significant fraction of the mechanical resonance frequency. With such extremely large coupling rates, it is possible to explore the “ultra strong coupling regime” (USC). The USC has not previously been observed in mechanical systems.