Exceptional precision of LC passive wireless sensors using nonlinear PT-symmetric configurations

Abstract

An inductor–capacitor (LC) passive wireless sensor is essential to physical, chemical, and biological sensing for scenarios where physical access is difficult. For emerging applications such as implantable, wearable, or other devices with geometrical constraints, the resulting small inductor coils weaken the strength of interrogation signals. It is therefore highly desired to enhance the sensitivity. Exceptional points (EPs) of parity-time (PT) symmetric LC systems featuring the linear loss and gain have been recently utilized for enhancing sensing. However, the EP sensing scheme might bring about fundamental resolution limits and noise enhancement. Here, we show, using PT-symmetric LC configurations of the linear loss and saturable gain, that the responsivity has a cube-root singularity distinct from a square-root singularity of the linear EP-scheme. The saturable gain allows the system to automatically achieve a steady-state oscillation, which eliminates the imaginary part of the eigenfrequencies and significantly suppresses the noise. As a result, the signal-to-noise ratio (SNR) is dramatically enhanced. Through an example of LC wireless flexible temperature sensors, we demonstrate the high SNR for the nonlinear PT-symmetric configuration. Our results resolve a debate on the effectiveness of the EP sensing scheme for LC sensors and pave the way for these types of sensor with exceptional precision.