Coherent Perfect Absorption and Amplifying System Enabled Ultrasensitive Ammonia Gas Sensing

Coherent perfect absorption and lasing (CPAL) phenomena offer an extraordinary platform for ultrasensitive sensing owing to their singular point with theoretically infinite quality factors. Yet, most radio frequency (RF) coherent perfect absorber-amplifier (CPAA) systems remain at the proof-of-concept stage, hindered by stringent impedance requirements at predesigned frequencies, susceptibility to slight component deviations, and strong parasitic effects during sensor integration. The scarcity of low-parasitic capacitive sensors suitable for high-frequency operation further limits practical deployment. Here, we present a strategy that precisely tunes the operating point of a CPAA system integrated with a capacitive sensor toward its theoretical singular point, thereby unlocking the superior detection sensitivity. By cooptimizing the CPAA configuration and sensor design, we achieved record-level sensitivities of 123 kHz/ppm and 0.2 dB/ppm for trace-level ammonia detection, corresponding to relative sensitivities of 0.06 and 0.42%/ppm, respectively. These values exceed the performance of conventional capacitive sensing (8.8 fF/ppm, or 0.04%/ppm) by up to an order of magnitude and significantly outperform state-of-the-art RF sensor platforms. Beyond ammonia sensing, this approach provides a generalized framework for enhancing the performance of the RF capacitive sensor, opening new opportunities for the practical realization of ultrasensitive chemical and biological detection technologies.