Metal nanoparticles have been used in applications long before the science of nanometersize materials have attracted much attention. Novel properties that are different from those of individual atoms or bulk materials are interesting not only from a fundamental research standpoint but also because of a wide variety of different potential applications [1-6]. Gold and silver nanoparticles were already used as coloring pigments in stained glass back in the Middle Ages. Faraday [7] first recognized that the red color is due to a “different form” of gold and postulated correctly that the color is caused by small gold particles without having any of the modern tools such as transmission electron microscopy (TEM). The intense color of metal colloids is due to the coherent excitation of the conduction-band electrons and is known as the surface plasmon absorption [8-10]. Mie [11] was able to theoretically model the plasmon absorption about a century ago by applying the Maxwell equations to describe the interaction between a metal nanoparticle with a known dielectric function and an electromagnetic field.