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ScienceWeek
MATERIALS SCIENCE: ON MOLECULAR PLASMONICS
The following points are made by Richard P. Van Duyne (Science 2004 306:985):
1) Today's information economy is driven by the electronics and photonics technologies, which use electrons and photons, respectively, to carry, store, and process information. An emerging branch of photonics, called "plasmonics", aims to use nanostructured materials that support "surface plasmons" for these purposes. Plasmonics can potentially achieve highly complex miniaturized devices by controlling and manipulating light on the nanometer scale (1-3). Several plasmonic devices -- including filters (1), wave guides (1,3), polarizers (4), and nanoscopic light source (5) -- have been demonstrated. However, for plasmonics to reach its full potential, active plasmonic devices such as switches and modulators will be required.
2) Andrew and Barnes (2004) have taken a step toward the realization of an active plasmonic device by combining thin polymer films containing molecular chromophores with thin silver films. The chromophores are the molecular equivalent of a Wi-Fi transmitter and receiver, transferring energy and hence information across the silver film with the help of surface plasmons.
3) When light interacts with a free-electron metal, such as a thin (10 to 200 nm) silver film, the metal surface electrons oscillate collectively and absorb, scatter, or re-radiate the incident photons. The resulting surface electromagnetic field propagates in the plane of the film (the x and y directions) with ranges of around 10 to 100 m, but decays exponentially in the z direction with a range of 200 to 300 nm. The field intensity in the z direction is amplified by a factor of 10 to 100 relative to the incident intensity. These propagating electromagnetic modes are properly termed surface plasmon polaritons, but are often referred to simply as surface plasmons.
4) The molecular plasmonic device constructed by Andrew and Barnes (2004) consists of two polymer layers, one containing donor (D) chromophore molecules and the other containing acceptor (A) fluorophore molecules. These layers are deposited on either side of a thin (30 to 120 nm) silver film to form a sandwich structure that is supported on a quartz substrate. The donors absorb incident light and transfer this excitation energy by dipole-dipole interactions to the acceptors. The latter then emit their characteristic fluorescence.
References (abridged):
1. W. L. Barnes, A. Dereux, T. W. Ebbesen, Nature 424, 824 (2003)
2. C. L. Haynes et al., J. Phys. Chem. B 107, 7337 (2003)
3. S. A. Maier et al., Nature Mater. 2, 229 (2003)
4. C. L. Haynes, R. P. Van Duyne, Nano Lett. 3, 939 (2003)
5. H. J. Lezec et al., Science 297, [820] (2002)
Science http://www.sciencemag.org
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Related Material:
OPTICS: ON SURFACE PLASMONS
The following points are made by W.L. Barnes et al (Nature 2003 424:824):
1) Surface plasmons (SP) are waves that propagate along the surface of a conductor, and they are of interest to a wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. Renewed interest in SPs comes from recent advances that allow metals to be structured and characterized on the nanometer scale. This in turn has enabled us to control SP properties to reveal new aspects of their underlying science and to tailor them for specific applications. For instance, SPs are being explored for their potential in optics, magneto-optic data storage, microscopy, and solar cells, as well as being used to construct sensors for detecting biologically interesting molecules.
2) SPs were widely recognized in the field of surface science following the pioneering work of Ritchie in the 1950s (1). SPs are waves that propagate along the surface of a conductor, usually a metal, and are essentially light waves that are trapped on the surface because of their interaction with the free electrons of the conductor (strictly speaking, they should be called surface plasmon polaritons to reflect this hybrid nature(2)). In this interaction, the free electrons respond collectively by oscillating in resonance with the light wave. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light constitutes the SP and gives rise to its unique properties.
3) For researchers in the field of optics, one of the most attractive aspects of SPs is the way in which they help us to concentrate and channel light using subwavelength structures. This could lead to miniaturized photonic circuits with length scales much smaller than those currently achieved(3,4). Such a circuit would first convert light into SPs, which would then propagate and be processed by logic elements, before being converted back into light. To build such a circuit one would require a variety of components: waveguides, switches, couplers and so on. Currently much effort is being devoted to developing such SP devices; one example is a 40 nm thick gold stripe that acts as a waveguide for SPs An appealing feature is that, when embedded in dielectric materials, the circuitry used to propagate SPs can also be used to carry electrical signals. Developments such as this raise the prospect of a new branch of photonics using SPs, sometimes called "plasmonics".
4) In summary: By altering the structure of a metal's surface, the properties of surface plasmons -- in particular their interaction with light -- can be tailored, which offers the potential for developing new types of photonic devices. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.(5)
References (abridged):
1. Ritchie, R. H. Plasma losses by fast electrons in thin films. Phys. Rev. 106, 874-881 (1957)
2. Burstein, E. in Polaritons (eds Burstein, E. & De Martini, F.) 1-4 (Pergamon, New York, 1974)
3. Hecht, B., Bielefeldt, H., Novotny, L., Inouye, Y. & Pohl, D. W. Local excitation, scattering, and interference of surface plasmons. Phys. Rev. Lett. 77, 1889-1892 (1996)
4. Pendry, J. Playing tricks with light. Science 285, 1687-1688 (1999)
5. Kneipp, K. et al. Single molecule detection using surface-enhanced Raman scattering (SERS). Phys. Rev. Lett. 78, 1667-1670 (1997)
Nature http://www.nature.com/nature
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Related Material:
ON THE PLASMON RESPONSE OF COMPLEX NANOSTRUCTURES
Notes by ScienceWeek:
The term "plasmon" refers in general to a quanta of waves produced by collective effects of a large number of electrons in matter when the electrons are disturbed from equilibrium. The concept arises from the theoretical treatment of free electrons in metals as an ionized gas (a "plasma"). Plasmons and phonons are examples of "collective excitation", a quantized mode in a many-body system produced by cooperative motion of the whole system as the result of interaction between particles.
The following points are made by E. Prodan et al (Science 2003 302:419):
1) The fabrication of materials on a nanoscale can be used to enhance and exploit properties that become stronger under conditions of reduced dimensionality. In metallic systems, the conduction electron charge density and its corresponding electromagnetic field can undergo plasmon oscillations. Because of the nature of the optical constants for noble metals, the charge oscillations can propagate along the surface (rather than vanish evanescently) at optical frequencies. These surface plasmons can be excited by incident light in a process that depends on the dielectric constant of the material at the metal's surface, an effect that is exploited in surface plasmon resonance spectroscopy. In particles of dimensions on the order of the plasmon resonance wavelength, this surface plasmon dominates the electromagnetic response of the structure.
2) Recent advances in the chemical synthesis of metal nanostructures have led to a proliferation of various shapes such as rods (1,2), shells (3-5), cups, rings, disks, and cubes. These developments, in addition to deep submicrometer lithographic methods for fabricating nanostructure grids and arrays, have provided the tools for realizing experimental studies of plasmon properties of metal nanostructures of arbitrary geometry. To fully exploit these new fabrication capabilities, accurate numerical methods for calculating the electromagnetic properties of nanoscale structures are essentially defining the new field of "plasmonics", providing an understanding of how to manipulate light at the nanometer scale with metal nanostructures as nano-optical components.
3) The authors describe a method by which the plasmon response of metal-based nanostructures can be understood as the interaction or "hybridization" of plasmons supported by metallic nanostructures of more elementary shapes. The plasmon hybridization picture can be used to describe the sensitive structural tunability of the plasmon resonance frequency of the nanoshell geometry as the interaction between plasmons supported by a nanoscale sphere and cavity. The authors suggest this simple and intuitive picture can also be used to understand the plasmon resonance behavior of composite metallic nanostructures of greater geometrical complexity. The authors suggest the plasmon hybridization picture is important because it provides the nanoscientist with a powerful and general design principle that can be applied, both qualitatively and quantitatively, to guide the design of metallic nanostructures and predict their resonant properties.
4) In summary: The authors present a simple and intuitive picture, an electromagnetic analog of molecular orbital theory, that describes the plasmon response of complex nanostructures of arbitrary shape. The model can be understood as the interaction or "hybridization" of elementary plasmons supported by nanostructures of elementary geometries. As an example, the approach is applied to the important case of a four-layer concentric nanoshell, where the hybridization of the plasmons of the inner and outer nanoshells determines the resonant frequencies of the multilayer nanostructure.
References (abridged):
1. N. R. Jana, L. Gearheart, C. J. Murphy, J. Phys. Chem. B. 105, 4065 (2001)
2. S. R. Nicewarner-Pena et al., Science 294, 137 (2001)
3. S. J. Oldenburg, R. D. Averitt, S. Westcott, N. J. Halas, Chem. Phys. Lett. 288, 243 (1998)
4. C. Graf, A. van Blaaderen, Langmuir 18, 524 (2002)
5. Y. Kobayashi, V. Salgueirina-Maceira, L. M. Liz-Marzan, Chem. Mater. 13, 1630 (2001)
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