Exceptional Point in a Microwave Plasmonic Dipole Resonator for Sub‐Microliter Solution Sensing

Abstract Microwave resonance sensing has attracted significant interest due to its promising potential in developing ultracompact integrated sensors. For biomedical sensing applications, it is required to realize high sensitivity to tiny volumes of solution using such long microwave wavelengths. Here, a chiral microwave plasmonic resonator, which can induce exceptional point (EP) state based on the coalescence of non‐Hermitian dipole eigenmodes, is proposed. The intrinsically geometric asymmetry leads to strong asymmetry of non‐Hermitian Hamiltonian and strong coupling between the two non‐degenerate dipoles, hence resulting in significantly enhanced frequency splitting signals. Benefiting from the subwavelength dipole resonances, the proposed resonator has advantages of the EP state's high sensitivity to tiny targets and strong plasmonic wavelength compression. Its sensing performance is experimentally validated by metal scatterer detection and glucose solution measurement. The minimum detectable metal scatterer features a diameter of λ₀/280 and a volume of 2.9 × 10⁻⁹ λ₀³ (λ₀ is free‐space wavelength). The solution volume under detection features a volume of 4.4 × 10⁻⁷ λ₀³ and the reporting limit for glucose reaches 0.26 µmol. These results envision a feasible route for trace‐amount biomedical sensing at the microwave frequencies.