Surface physics of ultrahigh-Q silicon nitride membranes

The last two decades have seen a dramatic improvement in the fabrication and performance of nanomechanical resonators. In particular, the quality factor (Q) of nanomechanical silicon nitride (SiN) resonators has been improved over 5 orders of magnitude, reaching now values close to a billion at room temperature. The aim of this work is to gain full understanding of the remaining damping mechanisms in such SiN resonators and to improve Q by another order of magnitude. Nanomechanical resonators with this level of coherence and force sensitivity constitute a significant step forward to enabling a set of visionary breakthrough experiments in fundamental physics. The drastic improvement of Q in nanomechanical SiN resonators, since the discovery of dissipation dilution more than a decade ago, has been solely based on device design optimization. In contrast, the actual underlying dissipation that is diluted has not been investigated in detail and has not seen any improvements. It has been shown that surface losses are ultimately limiting Q in SiN resonators. It is the objective of this project to study SiN surface losses and develop feasible strategies to eliminate these losses. As a result of our efforts, we hope to achieve, for the first time, quantum-limited scanning force microscopy with the recently developed membrane- AFM setup. This instrument will be employed for quantum sensing (electron spin detection) and quantum signal transduction (parametric coupling between different membrane modes). This proposal establishes a confluence of nanoscale physics, quantum sensing, and surface chemistry. The latter, while of enormous importance for many fields of science and industry, has received little attention as a means to improve the performance of quantum force sensors. We identify several highly promising paths to achieve surface passivation or functionalization of SiN optomechanical devices. Beyond the innovative device treatment, we introduce an entirely novel methods for characterizing the surfaces of SiN resonators. The team at TU Wien introduces ultra- sensitive photothermal UV-Vis and IR spectroscopy, which enables the chemical analysis of single surface monolayers. The team at ETH Zurich proposes a dielectric force microscope that utilizes the ultrasensitive mechanical resonator as a sensor for tip-surface interactions. With this instrument, we will gain access to nanometer- resolution information regarding surface layer thickness and composition, including potentially the identification of relevant two- level defects. In combination with more traditional methods of characterization, this method will allow us to probe deeper than ever before into the microscopic origin of the dissipation limiting a large class of nanomechanical devices. The proposed research will be conducted by Univ.-Prof. Dr. Silvan Schmid and Dr. Robert G. West from TU Wien and Dr. Alexander Eichler from ETH Zurich in close collaboration with Dr. John Abendroth from ETH Zurich.