Critical point-based wireless sensors enabling tiny perturbation detection

High quality factor and sensitivity are critical to wireless sensors targeting small perturbation detection. Although introducing parity-time symmetric dynamics promises enhanced performance, the symmetric scheme often yields limited sensing behaviors. In particular, its implementation is hindered by the inherent difficulties in decoupling capacitance- and coupling strength-induced responses, the requirements of delicately matching and/or tuning both gain and loss, and the need of strong coupling strength. Here, we report a concept of critical point (CP)-based wireless sensors that do not rely on balanced gain-loss configurations, delivering ultrahigh quality factor with an extended interrogation distance. Owing to a sharp and deep reflection dip, the CP-based scheme can resolve the change in coupling coefficient down to 1.92 × 10^-4 and features frequency-independent responses. Furthermore, the CP-based scheme allows for identification of tiny asymmetric capacitive perturbations as small as 2.5 × 10^-5 without requiring active tuning of the other parameters under weak coupling.