Intermodal coupling as a probe for detecting nanomechanical modes

Nanoelectromechanical systems provide ultrahigh performance in sensing applications. The sensing performance and functionality can be enhanced by utilizing more than one resonance mode of a nanoelectromechanical-systems device. However, it is often challenging to measure mechanical modes at high frequencies or modes that couple weakly to output transducers. In this paper, we propose the use of intermodal coupling as a mechanism to enable the detection of such modes. To implement this method, a probe mode is continuously driven and monitored using a phase-locked loop, while an auxiliary drive signal scans for other modes. Each time the auxiliary drive signal excites the corresponding mode by matching the mechanical frequency, the effective tension within the structure increases, which in turn causes a frequency shift in the probe mode. The location and width of these frequency shifts can be used to determine the frequency and quality factor of mechanical modes indirectly. Intermodal coupling can be used as a tool to obtain the spectrum of a mechanical structure even if some of these modes cannot be detected conventionally.