Nonlinear plasmonics: controlling light with light

Abstract

Many scientists believe that the future of signal processing and computing lies not within electronics but in the field of subwavelength optics. Controlling light at sub-wavelength scales, forbidden by traditional far-field optics, relies on surface plasmon techniques. Surface plasmon polaritons (SPPs) are electromagnetic waves that exist on the surface of metals such as gold or silver. The interaction between the electromagnetic field and the free electrons in the metal helps confine SPPs tightly to the surface. The behavior of this wave can bemanipulated by nanostructuring either the surface or a nearby dielectric. By contrast, metallic particles (or nanoscale dielectric voids in ametallic host) can enhance the field by supporting localized plasmonic excitations that create superior field confinement. Both mechanisms can locally control the electromagnetic field, a pre-requisite for the development of such light-based circuitry and devices. Researchers have used plasmonics to design and test various circuit elements such as waveguides, mirrors, lenses, and resonators based on nanostructured metal films.1 These passive plasmonic systems can provide some of the essential components for signal processing and optical circuitry, but creating optical analogs to electrical circuits requires active devices. In order to directly make plasmonic components, scientists commonly propose two approaches, both based on themodification of the refractive index near the metal surface. The first uses the electro-optic effect exhibited, for example, by liquid crystal molecules. Here, the refractive index depends on the molecular alignment, which can be modulated using an electric field. Hybrid electro-optic devices are promising since electronic and photonic signals can be transferred, processed, and interconnected in the same metallic circuitry. A second method relies on dielectric materials with a large Kerr nonlinearity, or a refractive index that depends on the intensity of the illuminating light. Figure 1. (left) Modulation of transmission by an array of gold nanorods (perpendicular to the substrate) and (right) a hole array in a thin gold film. Both were coated with poly-3BCMU (blue in schematic) and pumped with control light at 514nm.