A variable refractive index sensor based on epsilon-near-zero spatial selection structure and its potential in biological detection

Abstract

In this paper, a spatially selective (SS) structure is investigated through the critical jump characteristic when the permittivity of the epsilon-negative (ENG) materials approaches zero. In the range where the incident frequency of the TE wave is 5.1–5.6 c d−1 and that of the TM wave is 5.0–5.7 c d−1, the transmittance exceeds 0.85 and the squareness factor (SF) surpasses 0.9. By adjusting the electronic plasma frequency to affect the position of epsilon-near-zero (ENZ) frequency, a wide-angle control of the SS characteristic extent is produced. Using the fragility of the critical angle to the background analyte medium, a variable refractive index sensor is proposed. When the electronic plasma frequency is continuously altered, the analyte with the measurement range of 1.1–1.3, 1.3–1.5, 1.5–1.7, or 1.7–1.9 can be detected. The corresponding sensitivity is 57.1 degrees/RIU, 48.8 degrees/RIU, 40.0 degrees/RIU, or 32.8 degrees/RIU. The elongation in the thickness of ENG materials contributes to the improvement of the SF, executing the discrimination of Vibrio cholera, E. coli, and Shigella flexneri. The effects of losses on device performance are also briefly considered. Different from previous studies, the proposed SS selective structure adopts a new ENZ mechanism, can simplify the design of the structure to a certain extent and has a possibility in the design of highly sensitive sensors.