Playing with Plasmons: Tuning the Optical Resonant Properties of Metallic Nanoshells
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Playing with
Plasmons: Tuning the Optical Resonant Properties of Metallic Nanoshells Naomi Halas
Abstract Nanoshells, concentric nanoparticles consisting of a dielectric core and a metallic shell, are simple spherical nanostructures with unique, geometrically tunable optical resonances. As with all metallic nanostructures, their optical properties are controlled by the collective electronic resonance, or plasmon resonance, of the constituent metal, typically silver or gold. In striking contrast to the resonant properties of solid metallic nanostructures, which exhibit only a weak tunability with size or aspect ratio, the optical resonance of a nanoshell is extraordinarily sensitive to the inner and outer dimensions of the metallic shell layer. The underlying reason for this lies beyond classical electromagnetic theory, where plasmon-resonant nanoparticles follow a mesoscale analogue of molecular orbital theory, hybridizing in precisely the same manner as the individual atomic wave functions in simple molecules. This plasmon hybridization picture provides an essential “design rule” for metallic nanostructures that can allow us to effectively predict their optical resonant properties. Such a systematic control of the far-field optical resonances of metallic nanostructures is accomplished simultaneously with control of the field at the surface of the nanostructure. The nanoshell geometry is ideal for tuning and optimizing the near-field response as a stand-alone surface-enhanced Raman spectroscopy (SERS) nanosensor substrate and as a surface-plasmon-resonant nanosensor. Tuning the plasmon resonance of nanoshells into the near-infrared region of the spectrum has enabled a variety of biomedical applications that exploit the strong optical contrast available with nanoshells in a spectral region where blood and tissue are optimally transparent. Keywords: bionanotechnology, nanoshells, nanophotonics, plasmonics, plasmons.
Introduction The optical properties of metal nanoparticles and their applications have been a topic of dramatically increasing interest over the last several years. Much of this interest is generated by the growing expertise in nanofabrication methods that enable more and more possibilities of realizing metallic nanostructures of controlled size and shape. This has led to a proliferation of nanoparticle shapes such as rods,1 disks,2 rings,3 cups,4 and cubes5 (see the introductory article 362
in this issue for illustrations of various shapes). Advances in chemical synthesis are complemented by developments in planar nanostructure fabrication based on cleanroom and hybrid techniques. These developments, along with the addition of fast, accurate numerical methods for calculating the electromagnetic properties of nanoscale structures,6–9 are providing us with useful building blocks for guiding, controlling, and manipulating light at the nanometer
scale, with metallic nanostructures as nanophotonic components. In the past several years, it has become increasingly apparent that by precisely controlling the
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