Exceptional precision of a piezoelectric resonance sensor at a cube-root singularity

Exceptional points (EPs) are non-Hermitian degeneracies, where eigenvalues coalesce and their corresponding eigenvectors collapse. Consequently, non-Hermitian systems biased at EPs can react much more drastically to external perturbations. Here, we establish and experimentally demonstrate a scheme using EPs to enhance the sensitivity of piezoelectric resonance sensors. By connecting an external inductance-capacitor circuit with a nonlinear negative resistance to the piezoelectric resonance sensors, they are transformed into a parity-time symmetric system featuring the linear loss and nonlinear gain, which supports a second-order EP. The nonlinear gain enables the system to achieve a steady-state oscillation, and the resonance-frequency shift owing to the adsorbed analytes follows cubic-root behavior rather than square-root behavior. As a proof of concept, a piezoelectric sensor covered with graphene oxide for humidity sensing has been fabricated and characterized. At relatively low humidity levels, a sensitivity enhancement of more than an order of magnitude is observed in comparison to that of a standard arrangement. Our results pave the way for piezoelectric sensors with unprecedented sensitivity.