Evanescent coupling in a waveguide fluoroimmunosensor

A key factor in the analysis of evanescently-coupled optical sensors, such as the planar waveguide immunosensor analyzed here, is the efficiency of coupling between the optical waveguide modes and the fluorescent sources located on the surface of the waveguide. This is an important parameter in determining the sensor's sensitivity to the analyte. We calculate this efficiency for several different sensor configurations using the finite-difference time-domain (FDTD) numerical technique, and find that the efficiency of one-way coupling can vary widely depending upon the fluorescent source polarization, phase, and distance from the surface, as well as the waveguide mode number and thickness. In particular, we find that when the layer containing the fluorescent molecules is uniform in refractive index, the coupling efficiency is larger than when the local environment possesses an irregular index.