FEM-based method for the simulation of dielectric waveguide grating biosensors

Abstract

Label-free optical biosensors are important tools to study the kinetics, interaction and presence of (bio) chemical compounds in various fields such as biotechnology, pharma, diagnostics as well as environmental and food quality monitoring. Systems based on planar optical waveguides with input/output grating couplers are of interest as they offer multiple tuning parameters for the chip design and their high sensitivity. In the present paper, an algorithm based on the Finite-Elements Method (FEM) is proposed for finding the chip response and optimizing the sensitivity of the sensor system. Total field and scattered field coupled with the Transmission Line Transfer Matrix Method (TLTMM) are compared for the FEM. Unlike some widely used approximations, the impact of the grating depth, shape, duty cycle as well as losses and surface roughness are taken into account. Another advantage of the presented method is the possibility to implement a large part of the algorithm with commercially available FEM solver. Several practical situations are treated proving the validity of the approach against the Local Interference Method (LIME). The waveguide losses appear to be a decisive parameter for the chip design.