An exciting new direction is the introduction of active materials (gain) and the integration of these negative-index materials with semiconductor and other gain media. The challenges are large as a result of the short range of the interactions and the nonradiative losses introduced by the close proximity of the gain media to the metal films.

Impact

New and improved optical materials have always led to advances in optical systems. Currently, the first tentative steps at realizing these materials are under way. As always, the materials are too difficult to work with and too lossy to realize the benefits. However, these are very early days in this process, and it is clear on the basis of analogies with other major advances in optical characteristics that there will be many new capabilities associated with these hitherto-unavailable characteristics. Some promising directions include nonlinear optics, subwavelength cavities and field concentration for both sources and detectors, imaging at scales much less than a wavelength, negative dispersion and dispersion compensation, and many others. These are discussed at length in later chapters in this report.

To date, most of the work on metamaterials has focused on the fabrication and demonstration of homogeneous materials. Recently, the Duke group demonstrated an inhomogenous metamaterial lens by systematically varying the structure of the metamaterial elements (Driscoll et al., 2006). Because the lens is fabricated with only few metamaterial layers, it is much more lightweight than traditional approaches. In another set of experiments, the same group has demonstrated the “cloaking” of electromagnetic radiation by arranging an inhomogenous array of metamaterial elements in concentric rings around an object (Schurig et al., 2006). These experiments point to exciting new directions for metamaterials and confirm the hypothesis stated above—new materials lead to new functionality and to new applications.

The enhancement associated with subwavelength apertures will be of particular importance in mid-and long-wave infrared applications such as focal plane arrays. Room-temperature IR detectors are either very noisy as a result of large thermal dark currents in narrow band-gap semiconductor materials or very slow as in microelectromechanical systems (MEMS)-based microbolometers because of the thermal response of the isolated materials. In both cases, plasmonic antenna concepts offer revolutionary new capabilities. The dark current scales with the detector area and the noise scales as the square root of the area; thus, the figure of merit is the relative signal for a small detector versus a large-area detector divided by the square root of the area ratio. For microbolometers, the speed scales directly as the area (capacitance and thermal time constant) of the small elements. Box 2-1 and Box 2-2 provide examples of optical system advances made possible by improved optical materials.

PLASMONICS

Plasmonics is a subfield of nanophotonics concerned primarily with the manipulation of light at the nanoscale, based on the properties of surface plasmons. Plasmons are the collective oscillations of the electron gas in a metal or a semiconductor. Rigorously, the plasmon is the quasi-particle resulting from the quantization of plasma oscillations, a hybrid of the electron plasma and the photon. Although plasmons are quantum mechanical in nature, their properties, most specifically with respect to the coupling of light to plasmon oscillations, can be described rigorously by classical electrodynamics. Surface plasmons (SPs) are the electromagnetic waves that propagate along metallic/dielectric interfaces; they can exist at any interface, and for any frequency region, where the complex dielectric constants of the media constituting the interface are of opposite sign and the sum of the dielectric constants are negative. SPs are supported by structures at all length scales. They largely determine the optical proper-



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