The simplest way of optically detecting metal nanoparticles is via the
light they scatter [63]. By tuning the probing wavelength to the plasmon res-
onance, one considerably improves selectivity against non-metallic objects,
and one can access sizes down to a few tens of nanometers [12, 58, 59]. As
mentioned, the main drawback of this method is that the scattered intensity
decreases steeply (as R 6) for small particles. Despite the large index contrast
between the metal and its surroundings, scattering of a single particle smaller
than about 30 nm can no longer be discerned from the scattering of other (of-
ten numerous) scatterers.