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

Electromagnetic (EM) resonance sensing technology enables real-time and label-free detection capabilities, holding considerable practical value and development prospects in biomedicine and environmental monitoring. In recent years, more efforts have focused on enhancing sensitivity and reducing detection limit. For instance, optical microcavity sensing technology have achieved single-molecule detection capability.

 

However, optical resonance sensing relies on complex and bulky optical path systems, imposing highly stringent requirements on noise reduction and vibration isolation. Meanwhile, portable and Internet of Things (IoT)-enabled microwave sensors and sensing systems have emerged, which are expected for home-based diagnostics and large-scale screening. However, achieving breakthroughs in microwave resonance sensing remains challenging due to the large disparity between microwave wavelengths and detection targets.

 

Spoof localized surface plasmons (SLSPs) based on artificial EM microstructures incorporate multiple sensing enhancement mechanisms, offering significant potential to overcome the bottleneck in microwave sensing technology. On the one hand, the SLSP mode can significantly enhance wave-matter interaction within a deep-subwavelength scale. On the other hand, the design flexibility of artificial EM microstructures provides an effective platform to explore novel EM resonance modes. Among them, exceptional point (EP) state in non-Hermitian systems has garnered significant interest due to its trace-amount sensing capabilities. Studies have verified that combining SLSPs with EP state can achieve synergistic sensitivity enhancement. However, their electrical sizes are typically limited to the subwavelength regime. The EP state at extreme EM scales remains unexplored due to the lack of effective coupling modulation methods.

 

In a new paper published in Light: Science & Applications, a team of scientists, led by Xuanru Zhang and Tie Jun Cui from the State Key Laboratory of Millimeter Waves, Southeast University, China, proposed a deep-subwavelength EP state in a pair of microwave plasmonic resonators each with an electrical size of 1/50 wavelength based on the antibonding and bonding modes. The pronounced coupling modulation is achieved via controllable escaping decay channels. The proposed EP state enables synergistic sensing enhancement by combining the non-Hermitian effect with the deep wavelength compression of SLSP resonance unit. The excellent sensing performance has been experimentally validated in two scenarios: contactless scatterer detection and nanomolar glucose sensing (Figure 1).

 

Figure 2 demonstrates the regulating process of the deep-subwavelength EP state based on the antibonding and bonding modes. As the rotation angle α increases, the transmission spectrum, along with the real parts and imaginary parts of the complex frequencies in the coupled-SLSP system, evolves and simultaneously coalesces at the EP state. A further increase in the rotation angle leads to resonance point (RP) state, characterized by degenerate real parts and split imaginary parts. Full-wave electromagnetic simulations and coupled-mode theory reveal that this significant coupling modulation capability derives from the controllable escape decay during the rotation dynamics. Meanwhile, the electrical field of the EP state is concentrated in one SLSP unit (inset of Figure 2b and Figure 2c), achieving significant EM enhancement within an extremely small mode volume of π/10⁶ λ₀³ (where λ₀ is the operating wavelength), thereby enhancing the trace-amount sensing capability. Conversely, the electrical field of the RP state is alternatively concentrated in each SLSP unit in each half period.

 

Figure 3 demonstrates the experimental sensing performance using the proposed EP state. The measurable field distributions using near-field mapping provides a practical method to define the EP state in experiments (Figure 3a and Figure 3d). Statistical analysis of the sensing performance demonstrates that the proposed EP state enables contactless detection of scatterer with an electrical size of 1/1600 wavelength (Figures 3f-h), and a detection limit of 50 nmol for glucose sensing (Figures 3i-k). Furthermore, the proposed EP state maintains a higher signal-to-noise ratio compared to the non-EP counterparts, owing to the significantly enhanced sensitivity and only modestly increased noise (Figure 3h and Figure 3k).

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