Plasmonics: Metal-worthy methods and materials in nanophotonics
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troduction Discoveries of the mid-20th century have helped shape the character and quality of our modern technological landscape. The 1940s and 1950s brought remarkable breakthroughs in materials science ranging from the theory of dopants to the first imaging of individual atoms and identification of the double-helix structure of DNA. This era also produced the first transistor, the first silicon solar cell, and development of the first lasers. New discoveries about materials properties enabled new devices that in turn provided more insight into the fundamental nature of materials. Around this time, a graduate student at the University of Tennessee—Rufus Ritchie—began researching the energy losses of electrons passing through metallic films. Ritchie was fascinated by then recent experiments indicating that the energy loss spectrum was characterized by several equally spaced sharp lines—implying that electrons lose their energy in integral multiples of some fundamental unit. The results were not consistent with a free-particle treatment, in which the motion of electrons in a metal is assumed to be independent of all other electrons. David Pines and David Bohm at Princeton suggested that the energy loss spectra might be due to collective oscillations of the electron plasma in the metal.1,2 Ritchie coined this new elementary oscillation a “plasmon” and began to explore its energy and angle-dependent properties.3 By further
investigating the effect of the film thickness, Ritchie accounted for depolarization effects near surfaces and provided the first analysis of “surface plasmons.”4 His theoretical work laid the foundation for what is now the thriving field of plasmonics.5–8 As defined by Ritchie, surface plasmons are collective oscillations of conduction electrons at the interface between a conductor and non-conductor. Whether excited by electrons or photons, plasmons represent a “hybrid” electron-photon mode confined to a conductor; the electrons oscillate with an amplitude and phase reminiscent of light waves, but are bound to a conducting surface. Because of the dual electronic and photonic nature of surface plasmons, they are characterized by extremely high electric field intensities and extremely small mode wavelengths. Compared to electromagnetic waves in vacuum, plasmonic fields can be enhanced by more than two orders of magnitude.9 Further, their modal volumes can be reduced by more than two orders of magnitude.10 These two properties—extreme field enhancement and confinement— render plasmons a fascinating playground for photonics, allowing optical signals to be controlled on length scales comparable to electronic wavelengths. Since their discovery in 1957, surface plasmons have been exploited in applications as diverse as single-molecule surface-enhanced Raman spectroscopy,11–13 nanoscale optical modulators,14,15 high-efficiency solar cells,16,17 improved
Jennifer A. Dionne, Stanford University, Department of Materials Science and Engineering; [email protected] Harry A. Atwater, California Institute of Technology, Depa
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