Deep-subwavelength exceptional point in microwave plasmonic resonators for enhanced sensing.

Exceptional points (EPs) in non-Hermitian electromagnetic (EM) systems have been in the spotlight over the past decades due to their remarkable enhancement effects in sensing sensitivity. Here, we explore EPs at a deep-subwavelength scale, where pronounced coupling modulation is achieved via controllable escaping decay channels. This EP state is realized in a pair of microwave plasmonic resonators each with an electrical size of 1/50 wavelength based on the bonding and antibonding eigenmodes. It can concentrate the EM field into an extremely deep-subwavelength mode volume, accompanied by significant field enhancement, hence enhancing the trace-amount sensing capability. A high signal-to-noise ratio is also maintained, owing to the significantly enhanced sensitivity and only modestly increased noise. Experimental validation of the sensing performance is provided by contactless scatterer detection and nanomole-level glucose measurement. The smallest detectable contactless scatterer size is 1/1600 of the wavelength, and the detection limit for glucose reaches 50 nmol in amount of substance at the operating wavelength of 0.32 m. Our results reveal novel modulation mechanism in the deep-subwavelength EM regime, providing a broadened understanding of non-Hermitian EM systems. The nanomole-level microwave sensing experiments envision a new and promising route for label-free biomedical sensing.