Design of Polymer Optical Waveguides for the Detection of Azo Dye Water Contaminants

The design of single-mode optical waveguides made of SU-8 photoresist and functionalized with coatings of chitosan-based hydrogels to detect water-polluting azo dyes is presented through analytical and numerical analyses. Two possible waveguide configurations were evaluated in order to obtain the dimensions that allow the maximum sensitivity and facilitate their fabrication through direct laser writing (DLW) lithography method. The sensitivity of the waveguides was studied for TE and TM polarizations at NIR frequencies (λ=975 nm). We systematically explore how both, the hydrated and dehydrated states of the hydrogel receptor/cladding layer, and the waveguide’s dimensions, core width (2.0 < w < 3.0 µm) and height (h = 0.75 and 1.0 µm), influence the sensitivity. The results reveal that TM polarization yields higher sensitivity compared to TE one, particularly in thicker waveguides (h = 1.0 µm). Dehydration of the hydrogel cladding significantly enhances the interaction between the evanescent wave and the hydrogel, leading to a threefold increase in sensitivity which facilitates the detection of pollutant dyes such as acid blue 113 (AB113) or methyl orange (MO). Thus, dehydrated waveguides under TM polarization are highly effective for environmental and biomedical sensing applications, offering a promising approach for the detection of specific contaminants in aqueous solutions.