Sensitivity optimization of an evanescent field particle detector based on a silicon nitride waveguide

Optical sensors that utilize the evanescent field of an integrated waveguide are applied in a wide range of applications. Recently, evanescent field particle detectors based on dielectric strip waveguides were successfully used for the detection of small particles (phi< 1 mu m). We present optimizations of silicon nitride slab and strip waveguides based on numerical simulations, which maximize the evanescent field that interacts with the analyte such as particles. The fraction of the total light power that is transmitted in the evanescent region can be tuned by geometric parameters of the waveguide and the operation wavelength. We show that the optimum height of the slab waveguide scales linearly with the operation wavelength, which is in agreement with analytic results from literature. Moreover, linear correlations between the optimum waveguide geometry and wavelength could be derived for silicon nitride strip waveguides that are utilized for particle detection. The results for the optimum strip waveguide geometry are dependent on the target particle size. The derived geometries represent the optimum configuration for an evanescent field particle detector based on silicon nitride strip waveguides in order to exploit its full potential in terms of detection sensitivity. Enhanced sensitivities will be necessary to extend the detection range of evanescent field particle detectors down to small particles in the ultrafine regime (phi <= 100 nm).