Dynamics of molybdenum disulfide resonators coupled to acoustic and mechanical systems
Embargo Date
2027-01-31
OA Version
Citation
Abstract
Micro and Nanoelectromechanical systems (M/NEMS) have been studied extensively in the past few decades and their fabrication and characterization are well-established and employed in commercial devices. Two-dimensional (2D) materials are interesting candidates to be used as NEMS devices due to their extremely low mass, high flexibility, and high stiffness. While previous studies involved directly employing 2D materials as the sole material for nanomechanical resonators, there is still room for studying the aforementioned NEMS devices coupled to a non-2D material resonator. Integrating 2D materials with these well-studied traditional devices will enable us to utilize the unique properties of 2D materials in these established NEMS devices.
In this thesis, molybdenum disulfide (MoS2), one of the most famous 2D materials, is studied. An all-optical drive and detection scheme using two separate lasers is employed to characterize the dynamical behavior of resonators made from MoS2. Two cases of MoS2/NEMS hybrid devices are investigated; highly pressurized MoS2 resonators and mechanically coupled MoS2/Silicon Nitride (SiNx) hybrid resonators. In both of these cases, due to the introduction of a new resonator coupled to the system, the behavior of MoS2 is modified.
First, we investigate the change in mechanical behavior of MoS2 membrane resonators coupled to a gas with pressure as high as 1.3 MPa. By increasing the applied pressure across the membrane, the frequency of the resonator increases as expected. We developed a model in which the effect of the compression and expansion of the gas is accounted for as an extra spring in the system. It is found that the theoretical results from this model match the experimental data. The results of these experiments give us a better understanding on how the mechano-acoustic coupling can be utilized to tune the behavior of the 2D material resonators.
Next, we study the linear dynamics of MoS2/SiNx coupled resonators. Linear coupling of MoS2/SiNx resonators is also evident from the existence of avoided crossing in frequency dispersion curves from which we extract the coupling strength between various modes of MoS2 and SiNx. Measuring the inter-modal coupling strengths will be helpful during the design process of 2D material/NEMS sensors.
Finally, we investigate nonlinear effects in the coupled MoS2/SiNx resonators. By driving the system in a nonlinear regime we observed the appearance of higher-order harmonics, wave mixing, and frequency combs. The nonlinearity also extends to the damping of the system as well. Under certain optical inputs, the device can resonate without a driving force. We observed that by increasing the power of the optical probe, the resonance amplitude will decrease (i.e. laser cooling occurs). Again, we conclude that the origin of this behavior is the nonlinear nature of the damping term in the system’s model. The study of coupling nonlinearities paves the way for employing coupled resonators in applications such as signal processing, computing, developing phononic lasers, and cooling resonators down to the quantum ground state.