Modeling SPR sensors with the finite-difference time-domain method

In sensors employing surface plasmon resonance (SPR), the uniformity of the metal film and the wavefront structure of the incident beam have an effect on angular sensitivity, yet most modeling methods are not capable of considering inhomogeneous layers or nonplanar excitation beams. We have applied the numerical electromagnetic method of finite-difference time-domain to this problem. To correctly model the time-domain behavior of the metal's electron oscillations, we add a kinetic force equation consistent with the Drude free electron model. We have analyzed an SPR configuration consisting of an illuminating beam of finite size (approximating a focused beam) incident onto a smooth silver film, and have obtained Poynting vector plots and reflectivity data. We find that the angular width of the near-field reflectivity minimum is in reasonable agreement with an extension of planewave theory using an angular spectrum approach. We have also analyzed a model of a rough metal film, and find that the reflectivity curve is broadened and shifted, and that local electric fields are enhanced near the metal edges.