S4: Stanford Stratified Structure Solver (http://fan.group.stanford.edu/S4/) A program for computing electromagnetic fields in periodic, layered structures, developed by Victor Liu (victorliu@alumni.stanford.edu) of the Fan group in the Stanford Electrical Engineering Department. See the S4 manual, in doc/index.html, for a complete description of the package and its user interface, as well as installation instructions, the license and copyright, contact addresses, and other important information.