Intermodal Coupling as a Probe for Detecting Nanomechanical Modes

The performance of sensors based on nanoelectromechanical systems (NEMS) can be further enhanced by utilizing more modes of the resonator than just the fundamental. Often, though, it is challenging to detect higher-order mechanical modes, as their frequencies are too high, or they couple weakly to output transducers. The authors develop a technique for detection through intermodal coupling: When higher-order modes are excited, they induce a shift in the fundamental mode's frequency, which can be detected by a sensitive phase-locked loop circuit. Thus the spectrum of a mechanical structure can be obtained, even if some modes cannot be detected conventionally.