Ultrasensitive Sensing Enabled by Self-Dual Singularities in Parity-Time Symmetry

Coherent perfect absorber (CPA)-laser points are peculiar spectral self-dual singularities in non-Hermitian systems respecting the parity-time (PT) symmetry. This singular point possesses a unique feature that simultaneously exhibits two different eigenmodes within a single system, maintaining an exceptionally high-quality factor with great potential for diverse applications. However, despite the recent primary proposal of utilizing its CPA mode as the sensing mechanism, the capability of CPA-laser-based (actually amplifier in electronics) sensing systems has not yet been exploited thoroughly. Here, we propose that both the variation of output energy intensity and drastic frequency shift of the CPA-amplifier (CPAA) systems subject to tiny perturbations can be utilized as the sensing principle. We theoretically demonstrate that the CPA mode is more preferred when detecting the output energy intensity, and perturbation occurring to the loss component will lead to a larger modulation depth than that occurring to the gain component. On the other hand, the amplifying mode can be utilized to track the CPAA frequency shift. The analytical results indicate that this radio frequency (RF) CPAA sensing system can detect capacitive perturbation down to several femtofarads, with a sensitivity of 25 dB/pF and 10 MHz/pF, and is verified experimentally. Further, we practically integrate a high-frequency capacitive pressure transducer into the CPAA sensing system. The system detects subtle pressure changes of 0.4 Pa, corresponding to a 100-kHz frequency shift or 2-dB variation in the system’s output intensity. Our results may have a profound impact on practically implementing CPAA sensing systems as the next-generation ultrasensitive sensors.