Pattern-Free Tunable Angular Propagation Characteristics Based on Edge States and Its Potential in Sensing

In this article, by combining the epsilon-near-zero (ENZ) edge state of the InSb material and the photonic bandgap (PBG) edge state of photonic crystals (PCs), the angular transparent window (ATW) in a specific angle range is excited. The ATW is pattern-free, meaning that the same windows can be excited without being affected by polarization. The left edge of ATW is controlled by the refractive index of the PC dielectrics, while the right one can be adjusted by the temperature, and the two edges can be independently regulated. ENZ edge state maintains linear variation in both frequency and angle domains, which provides the possibility for the sensing design in two freedoms. Frequency-domain detection can be used for a large measurement range while angle-domain detection is equipped with extremely high sensitivity. In addition, the proposed structure has the capability of multivariable sensing to measure different physical quantities. For temperature detection, the maximum measurement range and the maximum sensitivity are 260-350 K and 26.678°/K, respectively, while for refractive index sensing, the corresponding values are 1.3-1.9 and 97.9°/RIU, respectively. In addition, due to the high sensitivity of the angular refractive index sensor, the proposed structure can also be utilized to detect citrate solution. The sensors of two freedoms based on the ENZ edge state provide the possibility to realize the detection of a large range, high sensitivity, and multiple functions at the same time.