Enhanced sensitivity of a film bulk acoustic resonator (FBAR) humidity sensor by a nonlinear parity-time (PT) symmetric system

Variations in surface-adsorbed mass linearly shift the frequency of piezoelectric sensors, thereby enabling precise measurement of target molecules and facilitating applications in biological and gas sensing. The growing demand for high-precision piezoelectric sensors has spurred research into enhancing sensitivity, including the optimization of sensing materials and resonant modes. This study introduces an approach to enhancing the sensitivity of piezoelectric sensors by utilizing the Exceptional Point (EP) in Parity-Time (PT)-symmetric systems. By capacitively coupling an external LC circuit with a nonlinear negative resistance to the piezoelectric sensor, we transform the sensor into a PT-symmetric system with linear loss and nonlinear gain. Both theoretical and experimental results reveal that when an asymmetric perturbation is applied to the loss resonator, the eigenfrequency of the nonlinear PT-symmetric system biased at the EP exhibits a cube-root singularity. The nonlinearly saturated gain can eliminate the imaginary part of the eigenfrequency and suppress noise. A Film Bulk Acoustic Resonator (FBAR) sensor coated with a cellulose acetate (CA)/graphene oxide (GO) composite was fabricated and characterized. An experimental PT-symmetric system was constructed using the CA/GO-FBAR sensor. The humidity perturbation experiments demonstrate that the PT-symmetric FBAR sensor system achieves a significant sensitivity enhancement under small perturbations. This novel approach overcomes the traditional limitations related to materials and structures for sensitivity enhancement. Furthermore, the concept of PT symmetry could be extended to other piezoelectric sensors for biological and gas sensing, thereby enabling high-sensitivity detection of trace target molecules via the integration of diverse sensitive materials.