Harnessing Evanescent Wave Interaction for Enhanced Optical NO2 Detection with Carbon Nanotube-Coated Side-Polished Fiber

Gas monitoring systems based on side-polished optical fibers (SPFs) coated with functional nanomaterials are gaining growing attention for their diverse applications. The response of these sensors is commonly interpreted in terms of absorption modulation of the sensing layer. Here we demonstrate that, in such systems, gas-induced reshaping of the guided optical mode can dominate the transduction mechanism and even reverse the sign of the optical response. Using SPFs functionalized with single-walled carbon nanotube (SWCNT) thin films, we observe a pronounced polarization- and film thickness-dependent response to NO2 exposure. This behavior contradicts established mechanisms of SWCNT-NO2 interactions and cannot be explained solely by gas-induced changes in intrinsic nanotube absorption. Numerical analyses show that an accurate description of the response requires accounting of modification of fiber mode profile induced by alterations in the SWCNT refractive index. The identified mechanism provides a general framework for designing fiber-optic and integrated photonic sensors based on evanescent-field interactions with functional nanomaterials.