Harnessing Evanescent Wave Interaction for Enhanced Optical NO2 Detection with Carbon Nanotube-Coated Side-Polished Fiber
Evanescent-wave photonic sensors employing nanomaterial-coated waveguides are 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 side-polished optical fibers covered with single-walled carbon nanotube (SWCNT) thin films, we observe a pronounced polarization- and thickness-dependent response to NO2 exposure. For transverse electric polarization, the optical response decreases with increasing film thickness and changes sign for thicker coatings, whereas for transverse-magnetic polarization, the response increases monotonically. Numerical modeling reveals that modulation of the SWCNT complex refractive index redistributes the evanescent field, alters the mode-matter overlap integral, and controls propagation loss. These results demonstrate that evanescent-wave sensor behavior is governed not only by intrinsic material sensitivity but also by mode reshaping induced by nanomaterial coatings. The identified mechanism provides a general framework for designing fiber-optic and integrated photonic sensors based on evanescent-field interactions with tunable nanomaterials.