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 ∼ 4 × 10 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 Si 3 N 4 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...

Engineering the Dissipation of Crystalline Micromechanical Resonators

“…The non-uniform residual stress σ r of hetero-epitaxially grown 3C-SiC [23] allows us to investigate the dependence of the mean stress on film thickness σ(h). To find the relation σ(h), we measure the fundamental resonance frequency ω m of each fabricated trampoline from its noise power spectral density with an optical heterodyne detection system operating in vacuum (P ∼ 10 −6 mbar) as reported previously [30,34]. The quality factor Q is determined via a ringdown measurement after applying an impulse to the trampoline as shown in Fig.…”

“…In high-vacuum environments, the external energy dissipation is exclusively attributed to clamping losses Q −1 clamp [29], which we de- [21] and intrinsic friction in the volume of the material (Q −1 vol ) [37], and is detailed in Sec. IV C. Thermoelastic losses in highly stressed trampoline resonators were calculated [30] using existing models [38], and are neglected in the rest of this work due to their small contributions in thin resonators [39]. The total quality factor is given by…”

“…The radius of the clamping point is engineered to optimize the ratio of elastic energy stored as elongation as opposed to bending which is a source of losses [12,29], enhancing the Q by diluting the dissipation for the fundamental vibration mode. In this approach, two counteracting and competing mechanisms are present; the increased material volume at the widened clamps stores a larger amount of bending energy, while the increased rigidity reduces the overall bending [30,31]. The elongation to bending ratio converges to a maximum value for an optimum radius of curvature R. According to our finite element simulations, the optimal radius of curvature for our crystalline resonators is R = 30 µm, predicting a Q about four times larger compared to rigid clamping (R = 0).…”