Measuring the speed of a surface plasmon

This paper presents experiments using an 800-nm pulsed laser and a streak camera with 2-ps resolution, which is implemented with a two-fiber differential measurements permitting a 0.2-ps resolution. With this technique, we measure the speed of the propagation of the surface plasmons. This velocity was found to be approximately(0.57 ±0.19)c, about 61% the value expected from simple theory (i.e., about0.94c) The former result may clarify what is going on at the metal vacuum interface excitations, and has been measured by a direct method without the need for any theory.