Study of soft clamping in ultracoherent nanomechanical resonators

Abstract

Strained silicon nitride nanomechanical resonators have received a lot of attention in recent years, as the small mass, high coherence times and exceptionally high quality factors (Qs) open up a broad range of applications. Increasing Qs reduce the intrinsic force noise and therefore pave the way for high sensitivity force sensors. In the field of quantum optomechanics, a high mechanical Q×f product is sought after for the ability to perform quantum experiments at room temperature. In this thesis, various properties of strained silicon nitride resonators will be studied. On the one hand, investigation into mechanical Q enhancement via two different approaches and on the other hand, the potential applicability of ”soft-clamped” high-Q resonators for photothermal detection. The whole process from finite element method simulations and photomask design, to fabrication and characterization will be explained in the subsequent chapters.In the first part, the limits of reducing bending losses for strings with engineered clamps was systematically investigated. The idea behind this approach is to localize the stress to the center of the string, while reducing the bending at the clamping. Al- though slight increases in Q could be observed, recently published results can however not be explained. As it was shown that surface losses are the ubiquitous loss mechanism for nanomechanical resonators, the next step was to decrease surface losses to a minimum. This was done by employing an ultrahigh vacuum chamber (UHV) and using ”soft-clamped” membranes embedded in a phononic crystal (PnC) structure. The PnC creates a phononic bandgap, which allows the membrane’s modes to only penetrate into the surrounding area evanescently, hence reducing the bending at the clamping enormously. Using the UHV chamber, Q×f products could be enhanced up to a factor of 20 when compared to the measurements performed in high vacuum. Additionally it will be shown, that annealing inside the UHV chamber can increase Q up to a factor of 17 compared to the initial level. Finally a potential novel application of PnCs was investigated. The low noise and reduced thermal conductivity, as a consequence of the perforation of the membrane, promise a high thermal responsivity. This can be utilized for trace analysis of minute samples via photothermal heating and the consequential detuning of the membrane’s resonance frequency.