Materials Screening and Applications of Plasmonic Crystals
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stract Surface plasmon polaritons are responsible for various optical phenomena, including negative refraction, enhanced optical transmission, and nanoscale focusing. Although many materials support plasmons, the choice of metal for most applications has been based on traditional plasmonic materials, such as Ag and Au, because there have been no side-by-side comparisons of different materials on well-defined, nanostructured surfaces. This article will describe how a multiscale patterning approach based on soft interference lithography can be used to create plasmonic crystals with different unit cell shapes—circular holes or square pyramids—which can be used as a platform to screen for new materials. The dispersion diagrams of plasmonic crystals made from unconventional metals will be presented, and the implications of discovering new optical coupling mechanisms and protein-sensing substrates based on Pd will be described. Finally, the opportunities enabled by this plasmonic library to dial into specific resonances for any angle or material will be discussed.
Introduction Broadly defined, plasmonics encompasses the science and application of metal structures that can manipulate light at the nanoscale. This rapidly growing field is making inroads into the materials community because of a combination of three synergistic factors—Make, Measure, Model—that we have designated as the 3Ms Principle.1 Advances in nanostructure synthesis, fabrication, and processing, innovations in measurements for determining structure-function relations, and improved capabilities of theory and simulation have enabled surface plasmons (SPs) to impact diverse applications.2,3 SPs are collective charge oscillations that form when light interacts with free electrons at a metal-dielectric interface.4 Because of their concentrated electromagnetic fields and near-field localization, SPs have been used
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in subwavelength focusing and nanoscale waveguiding,5 in achieving negativeindex refraction,6,7 and in label-free biological and chemical sensing.8 Nearly all of these phenomena have relied on noble metals Ag and Au; however, plasmons with particular characteristics are required for different purposes. For example, ultraviolet (UV) resonances are favorable for nanolithography,9 while near-infrared (NIR) plasmons are desirable in photothermal cancer therapy.10 Moreover, narrow SP peaks can exhibit higher sensitivities in sensing,11 but broadband excitation is necessary to enhance photo-absorption in solar cells.12 There is currently a gap in screening for other materials so that not only can the prospects of SPs be broadened but also current applications can be optimized.
This article will describe how a materials science approach to plasmonics offers new opportunities to explore unconventional materials that otherwise would not have been pursued because of traditional bias. Our perspective is that plasmonic materials and structures should be designed to meet specific scientific and technological goals rather than limiting potential prospects be
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