Plasmon transverse

Surface plasmon-polaritons (SPP), also referred as to surface plasma waves, are special modes of electromagnetic field which can exist at the interface between a dielectric and a metal that behaves like a nearly-iree electron plasma. A surface plasmon is a transverse-magnetic mode (magnetic vector is perpendicular to the direction of propagation of the wave and parallel to the plane of interface) and is characterized by its propagation constant and field distribution. The propagation constant, P can be expressed as follows ... 

Nevertheless, certain collective excitations can occur in the condensed phase. These may be brought about by longitudinal coulombic interaction (plasmons in thin films) or by transverse interaction, as in the 7-eV excitation in condensed benzene, which is believed to be an exciton [12]. Special conditions must be satisfied by the real and imaginary parts of the dielectric function of the condensed phase for collective excitations to occur. After analyzing these factors, it has been concluded that in most ordinary liquids such as water, collective excitations would not result by interaction of fast charged particles [13,14]. 

Plasmon-phonon coupling represents mixing of two quasi-particles. The coupling of three quasi-particles has also been observed. The term plasmariton was used by Alfano 45) for a coupled state of a TO phonon and a dressed photon , namely, a photon surrounded by an electron cloud (a coupled state of a plasmon and a photon). The quasi-particle dressed photon is also called a transverse plasmon. Because the coupled state of a photon and a TO phonon has been termed polariton, a plasmariton can also be regarded as coupled state of a plasmon and a polariton. Earlier the term plasmariton was used in a more restricted sense, namely, when a partly transverse character of the plasmon is induced by an external magnetic field. 

In order to assemble gold nanorods (aspect ratio 2.4) by the click reaction, one batch of nanorods capped with 4-azidobutane-l-thiol, A, and another with hex-5-yn-l-thiol, B, were reacted in the acetonitrile-water mixture under standard conditions [12], The occurrence of the click reaction was established by infrared spectroscopy [13], The product of the reaction was investigated by electronic absorption spectroscopy and transmission electron microscopy. Fig. 1 compares the electronic absorption spectra of the gold nanorods before and after the click reaction between the A and B type nanorods. Isolated gold nanorods show transverse and longitudinal plasmon bands around 520 nm and 630 nm, respectively (see Fig. la). 

Fig. 9.2. Cartoon illustrating changes in the light scattering spectra when two noble metal nanoparticles are brought into close proximity to each other. The plasmon resonance splits into longitudinal and transverse modes, with scattering from the longitudinal mode more intense. This can be used as the basis for a plasmon ruler with application in single molecule biophysics...

The electronic absorption spectrum of metal nanocrystals in the visible region is dominated by the plasmon band. This absorption is due to the collective excitation of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being well-known [6-8]. The dependence of the plasmon peak on the dielectric constant of the surrounding medium and the diameter of the nanocrystal was predicted theoretically by Mie and others at the turn of the last century [9-12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7]. 

Fig. 9.11. Absorbance vs. time plot for cyanide dissolution at wavelengths corresponding to the transverse and longitudinal plasmon bands. Conditions were similar to set f in Figure 9.9. Reprinted with permission from [62]. Copyright (2002)...

Figure 34.1 Extinction (absorption) spectrum of gold nanorods and their transmitted electron microscopic image (inset). Cold nanorods (10 x 60 nm) showed peaks at 520 and 900nm corresponding to the transverse and longitudinal surface plasmon...

Anisotropic noble nanoparticle dispersions are very different in colour compared to dispersions of spherical particles. This is because the surface plasmon bands are more sensitive to particle shape than size [92]. All the metal nanorods have two absorbance maxima that correspond to the longitudinal and transverse plasmon bands. The longitudinal plasmon band strongly depends on the aspect ratio. For example, platelets have additional quadrupole bands [96]. Upon transition from nanorods to platelets, as the aspect ratio decreases, the longitudinal band is blue-shifted and the transverse band becomes broad due to overlap with the quadrupole band. In cubes, all three plasmon bands merge into a single band. In contrast, transition from nanorods to nanowires increases the aspect ratio, which produces a resultant red shift of the longitudinal band and a blue shift of transverse band [83]. 

Plasmon surface polaritons (PSPs) or surface plasmons are transverse magnetic waves that propagate along a metal-dielectric interface, their field amplitudes decaying exponentially perpendicular to the interface [29,30]. Their dispersion relation is given by... 

Figure 7 shows extinction spectra calculated for randomly oriented linear chains of 13-nm gold spheres with interparticle spacing 1.1 mn. It is the same model that has been considered in Ref [70]. We note two spectral resonances related to the transversal and longitudinal (red-shifted) plasmon excitations. Such properties are analogous to the randomly oriented metal spheroids [45, 72]. By contrast to Fig. 8 in Ref [70], we conclude that random chain orientations do not eliminate the red-shifted longitudinal resonance. Perhaps, the spectra of Fig. 8 from [70] were calculated with an insufficiently large multipole expansion order. 

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