Electromagnetic fields in the cutoff regime of tapered metallic waveguides

We calculate electromagnetic (EM) fields in tapered metal cutoff waveguides by solving a simple system of two coupled differential equations which describe the incremental local reflection of the waveguide mode as the diameter changes gradually. The results give physical insight into aperture probes used in scanning near-field optical microscopy (SNOM). The method is an easily installed tool for SNOM optimization.