Nanoplasmonics is the study of optical phenomena in the nanoscale vicinity of metal surfaces. At first glance, that operational definition may be puzzling, given the common presumption that electromagnetic radiation cannot be concentrated, focused, or confined to a region whose size is less than half the light’s wavelength. Indeed, it appears obvious that the minimum volume of an optical field localized in a cavity—bounded by three pairs of ideal mirrors, say—is (λ/2)3, where λ is the wavelength. That’s about 0.01 µm3. The question, therefore, is this: How can an optical field be localized seven orders of magnitude tighter, on the scale of 1 nm3?
The answer lies in what happens to nanoparticles made of conducting metal such as gold when they are subjected to an external optical wave, as sketched in figure 1. The particles should be smaller than the metal’s...
