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 success- fully used for the detection of small particles (0 < 1 μm). 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 evanes- cent 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 (o≤ 100 nm).
Detecting and classifying particles over a wide range of types and sizes is essential for precise air quality determination. In this study the use of optical waveguide-based particle detection is examined using finite element method (FEM) based simulations. The simulation model assumes a silicon nitride strip waveguide and is built up in 3D using the Comsol Multiphysics platform. The waveguide geometry parameters were varied to identify suitable geometries for single-mode wave guidance of the fundamental quasi-TE and quasi-TM modes. The geometries with their according effective wave indices are reported. Furthermore, the intensity and phase changes of the single-mode wave introduced by the presence of a particle are analyzed und the underlying physical effects are discussed for spherical particles of radii from 50 to 500 nm. The results show non-linear and non-monotonic behavior and give substantial input to understand basic particle interaction with waveguide structures. Furthermore, they provide helpful knowledge for designing waveguide-based particle detectors.