Design optimisation of rare earth metal doped polymer optical planar waveguide sensor for microplastics detection in water

In recent times, the issue of microplastic pollution has garnered worldwide attention as it poses threat to both the environment and wildlife, including human health. This work demonstrated a simulation of a rare earth metal-doped polymer optical planar waveguide sensor with a circular core design to realise future sensing applications of detecting microplastics in water by utilising the Wave Optics Module in COMSOL Multiphysics® software. The sensor design optimisation was performed by varying the cladding thickness from 0 to 5 μm at a fixed 10 µm circular core diameter. The simulation involved changing the refractive index (RI) of the analyte, which varied from 1.480 to 1.500 refractive index units (RIUs) with respect to the RI of microplastics, to assess the optimum waveguide design for sensitivity performance of the sensor. The results successfully showed that the sensitivity of the waveguide sensor increased with decreasing cladding thickness. The highest sensitivity of 2.75 × 10^–4 in dimensionless unit was achieved for 0 µm cladding thickness waveguide sensor design. This sensitivity is 10⁷ higher in magnitude than 5 µm cladding thickness, with an output sensitivity of 8.40 × 10^–11 in dimensionless unit. Considerably, incorporating the rare earth metal-doped polymer composite into materials with an optimised waveguide sensor design of 0 µm cladding thickness assures an excellent sensing potential for microplastic detection in water.