The polymerization catalyst for a polythiourethane-based optical material of the present invention includes a sulfonate represented by the following general formula. The method includes: lithographically forming a plasmonic nanostructure on a substrate; encapsulating the plasmonic nanostructure in high temperature resistant material; annealing the plasmonic nanostructure; and removing the high temperature resistant material to reveal the annealed plasmonic nanostructure. A head piece is mounted to the upper body portion, and has a leading edge which forms an acute angle with one side of the head piece such that when the leading edge is placed flush against the running board of the vehicle, the creeper may be positioned in the space formed by an open door of the vehicle and the running board without interference by the door.