FDTD modeling in the design of optical chemical sensor structures

The finite-difference time-domain method (FDTD) is a numerical technique for solving Maxwell's equations in a discretized space and time frame. It has been used extensively in the analysis of electrically large structures in the microwave domain, but has only recently been applied to optical problems. Because of memory limitations of computers used, the method is generally restricted to configurations which extend to the order of tens of wavelengths in three dimensions, or hundreds of wavelengths in two dimensions. Optical sensor structures, however, are of suitable size to be modeled with FDTD, and the advantages of the technique become pronounced especially in the design of chemical sensors which employ evanescence or near-field effects. Among these is the possibility to incorporate the molecular distribution of the analyte on the sensing surface as well as the optical characteristics of the sensor in the optical model. Although the computer resources required for arbitrary structures in three dimensions can limit the scope of analyses, it is relatively simple to use symmetry properties of the structures to relax these requirements. In addition, for planar structures, two-dimensional models are adequate for studying many aspects of sensor design. We have applied FDTD to the analysis of excitation and emission as well as to optimization of the design of planar waveguide fluorescence sensors.