Split Quartz Tuning Fork Sensors for Enhanced Sensitivity Force Detection

Quartz tuning forks (TFs) are often employed in dynamic-mode atomic force microscopy (AFM) as piezoelectric force sensors, to replace the usual AFM microcantilevers, especially in ultra-high vacuum or cryogenic environments. A sharp tip is attached to one of the fork prongs, to obtain atomic scale AFM resolution. We devise a novel TF design by splitting the electrodes of its two prongs, which are produced at the factory as connected to each other, in order to address each of them separately. In such way, the motion of the probe tip can be unambiguously measured, irrespective of the motion of the other prong, which conversely influences its measurement in standard TFs. Furthermore, attachment of the probe tip dramatically spoils the oscillator Q-factor, as it unbalances the two prongs of the TF, with consequent dissipation of energy through the fork holder, due to the motion of the center of mass (CM) of the system. The possibility to independently drive the two prongs of the split TF gives the opportunity to rebalance them just by electrical means, thereby restoring the original Q-factor, by stopping the CM motion. By modeling the split TF as a three-mass, four-spring system, its behavior can be accurately described. Our model is used to explore alternative operation modes with enhanced sensitivity to applied forces.