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 for decades for their remarkable enhancement effects in wave-matter interactions. Here, we explore EPs in an intermediate EM regime, where the electrical size is in deep-subwavelength scale while the phase retardation governs the modulation. The EP state is realized in a pair of coupled microwave plasmonic resonators, each with an electrical size of 1/50 operating wavelength. The highly confined plasmonic evanescent field, associated with asymmetric excitation dynamics, enables pronounced phase modulation capacity in a deep-subwavelength regime. The combination of non-Hermitian EP effect and deep-subwavelength field concentration leads to a dramatic enhancement in wave-matter interactions. Experimental validation of the EM performance is provided by contactless scatterer detection and nanomole-level glucose sensing. The smallest detectable scatterer size is 1/1600 of the wavelength, and the detection limit for glucose reaches 50 nmol at an operating wavelength of 0.32m. Our results reveal novel modulation mechanisms in the deep-subwavelength intermediate EM regime, providing a deeper understanding of non-Hermitian EM systems. The nanomole-level microwave sensing experiments envision a new and promising route for label-free biomedical sensing.