Twin lossy mode resonance on a single D-shaped optical fiber

This Letter presents the fabrication of dual lossy mode resonance (LMR) refractometers based on titanium dioxide (TiO₂) and tin oxide (SnO₂) thin films deposited on a single side-polished D-shaped optical fiber. For the first time, to the best of our knowledge, two independent LMRs are obtained in the same D-shaped optical fiber, by using a step-shaped nanostructure consisting of a first section of TiO₂ with a thickness of 120 nm and a second section with a thickness of 140 nm (120 nm of TiO₂ and 20 nm of SnO₂). Each section is responsible for generating a first-order LMR with TM-polarized light (LMR_TM). TiO₂ is deposited by atomic layer deposition and SnO₂ by electron-beam deposition. The theoretical results show that the depth of each of the resonances of the dual LMR depends on the length of the corresponding section. Two experimental devices were fabricated with sections of different lengths, and their sensitivities were studied, achieving values ∼ 4000 nm/refractive index unit (RIU) with a maximum of 4506 nm/RIU for values of the SRI between 1.3327 and 1.3485.