Ultrahigh Refractive Index Sensitivity With Lossy Mode Resonance in a Side-Polished Low-Index Polymer Optical Fiber

— This work presents a theoretical–experimental study of a refractive index (RI) sensor based on lossy mode resonances (LMRs) generated with a side-polished low-index polymer optical fiber. The theoretical part involves conducting simulations to determine the optimal coating thickness for achieving the first (1st) and second (2nd) LMR, as well as to determine their respective sensitivities. In the experimental part, a cyclic transparent fluoropolymer optical fiber is used, which is first polished and then coated with tin oxide (SnO₂) thin film to obtain the LMR. The simulated and experimental results exhibit a high level of agreement. In the visible light spectrum region, the sensitivities for the first LMR are, respectively, 11 300 nm/RI unit (RIU) and 15 973 nm/RIU within the RI range of 1.333–1.345. Similarly, for the second LMR, the sensitivities are, respectively, 520 and 467 nm/RIU within the same RI range. Furthermore, experiments conducted in the near-infrared light spectrum region show a record sensitivity of 57 200 nm/RIU, while the second LMR exhibits a sensitivity of over 5 000 nm/RIU within the RI range of 1.340–1.345. These results underscore the potential of utilizing cyclic transparent optical polymer (CYTOP) fiber in various applications requiring the detection in liquid samples, such as biosensors or chemical sensors.