Feedback delays for ultrasensitive sensing

Nanomechanical Cantilever Sensors (NMCS) have recently emerged as an effective means for label-free chemical and biological species detection. Their high selectivity, low cost, and easy mass production make them an enabling technology for micro- and nanodetection techniques. Sensitivity constitutes one of the most desirable characteristics of NMCS. However, sensitivity of current NMCS is predominately limited by their size. In other words, smaller sensors are more sensitive than larger ones. The detection of ultra-small masses or frequency variations is therefore obstructed by the availability of more advanced fabrication techniques that are capable of manufacturing smaller and smaller sensors. Even in that case, other issues such as noise, damping, and measurement difficulties become more evident. Therefore, techniques for sensitivity enhancement should be studied and implemented to allow for accurate and precise detection of even smaller parameter variations. Along that line of reasoning, we propose a simple, but effective concept to enhance sensitivity of NMCS. The proposed methodology is based on utilizing feedback delays to create a limit-cycle response whose amplitude is ultra-sensitive to frequency variations. In this paper, we explain, analyze, and derive the equations that verify the proposed concept. We then experimentally implement the sensitivity enhancement technique on a macrocantilever beam and demonstrate more than two-orders-of-magnitude sensitivity enhancement over the frequency-shift method.