Ultra-Sensitive Anti-Parity-Time-Symmetric Wireless Sensor Based on Unidirectional Couplings

The coupled LC resonators satisfying parity-time (PT) symmetry or anti-PT (APT) symmetry can be utilized as highly sensitive sensors if calibrating them at exceptional points (EPs) in advance. In practice, however, the existing VNA-free and wireless PT- and APT-symmetric systems are restricted by the vague EP position and the requirement of imaginary-value couplings, respectively. These issues are addressed by the proposed novel APT-symmetric wireless sensor, constructed by four coupled LC resonators. A pair of unidirectional and opposite real-value couplings is implemented to achieve the wireless sensing functionality. The property of different numbers of operating frequencies at the APT-symmetric and -broken states is utilized to calibrate the system at the EP. The time-domain working mode avoids using VNAs to sweep the frequency. The system behavior is analytically analyzed and validated through simulations. The simulated results show that the frequency deviation response to the resonant frequency perturbation $\left(\varepsilon_{\omega}\right)$ features an ultra-high sensitivity, $\sim\left|\varepsilon_{\omega}\right|^{0.5}$.