Plasmonics has fascinated people for centuries, although they didn’t always fully understand its unique effects. During the Middle Ages, for example, infusions of colloidal metal particles in glass produced wondrously brilliant colors for cathedral windows. The colors were due to the excitation of plasmon oscillation modes in the metal particles, creating absorption bands and producing unique colors in the transmitted light.

As discussed last month in the first part of this feature, plasmonic materials are structures that support surface plasmon oscillation modes. In a metal, the conduction electrons are essentially free to move, with little interaction from their respective nuclei because of Coulomb shielding effects. The electric field of an incoming lightwave induces polarization of the conduction electrons with respect to the much heavier ionic cores of the metal atoms. At the surface of the metal, a net charge is produced. The net charge difference on the surface then acts as a restoring force for the polarization. As a result, the electrons oscillate coherently. If the frequency of the incident electromagnetic field is resonant with the coherent electron motion, a strong absorption in the spectrum is observed...