Dielectric Characterization of Liquid Mixtures Using EIT-like Transmission Window

Electromagnetically induced transparency (EIT) is a destructive interference phenomenon which has been widely reported in both quantum and classical wave systems. In an EIT-like system, sharp transmission windows appear within stop bands, indicating potentials in sensor applications. In this work, we design a planar microwave circuit with an EIT-like spectrum for dielectric characterization of liquid mixtures. The sharp EIT-like transmission window relies on the strong coupling between an open-circuited stub and a split ring resonator, which are sensitive to surrounding environment. Binary liquid mixtures of water-ethanol and water-methanol are injected into the container fixed on the surface of the sensitive area. Complex permittivity of the liquid under test affects both the center frequency and the quality factor of the EIT-like transmission window. Thus, complex permittivity of liquid mixtures can be further retrieved from measured S21. Measurements together with theoretical and numerical analysis are discussed in detail and results match well with Debye function. We also analyze the achievable high sensitivity (5%) of the proposed EIT-like sensor when measuring solid samples with low permittivity dynamic range and conduct experiments. The proposed design is easy to fabricate and can be used to test various materials.