Localized surface polaritons

Surface polaritons can also exist on curved and closed surfaces, e.g., on spheres and ellipsoids. In the latter case, SPs are called localized and the 

17) The quantities can be found from the Fresnel equations (see Section 3.1.1). 

SP dispersion relation depends to a great extent on the surface geometry (see, e.g., (Raether 1988)). 

In the nonretarded limit (c oo) the SP modes of a sphere having the dielectric function 62(0 ) are given by the relation 

The field at the external sphere surface can be found from the boundary conditions (see Fig. 3.9) 

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Surface plasmons, or surface plasmon polaritons, are surface electromagnetic waves that propagate inside a metal along a metal/dielectric (or metal/ vacuum) interface their excitation by light is surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metal particles. 

Bozhevolnyi SI, Vohnsen B, Smolyaninov II, Zayats AV (1995) Direct observation of surface polariton localization caused by surface-roughness. Opt Commun 117(5-6) 417 23... 

Fig. 16.2 Localized surface plasmon polariton excited at the metallic tip apex...

The electromagnetic enhancement mechanism features the major contribution to the overall enhancement of SERS. It is based on the generation of an electromagnetic field at the surface of nanostructured metal surfaces due to the interaction of an incident electromagnetic field and the excitation of localized surface plasmon polaritons. To explain this phenomenon in more detail, a simple model can be used. A simple metal nanosphere with a size smaller than the wavelength of the incident light is considered for this purpose. This metal nanosphere is surrounded by a medium or vacuum with a dielectric constant Eq, and all appearing processes are assumed to be quasi-static. The dielectric constant inside the metal nanosphere is independent of the size of the sphere and is described as follows ... 

If an incident laser light Eo interacts with the metal nanosphere, a collective movement of the electrons against the atomic cores of the metals - a so-called surface plasmon - is induced. The interaction of the surface plasmons and the incident laser light causes the so-called localized surface plasmon polaritons, resulting in an evanescent electromagnetic field E y, which is emitted fi-om the nanoparticle. The signal intensity achieved for SERS depends on the absolute square of the emitted field Egy, which can be simplified written as follows ... 

There is another method of optical trapping, enabled by the advent of nanophotonics the subwavelength optical localization utilizing plasmonic nanocomposites. Some metal-dielectric stmctures ensure the possibility of light localization on a level much smaller than the operating wavelength. To this purpose they utilize the propagating or localized surface plasmons polaritons. 

A SPP is confined to the interface between the positive and the negative permittivity part and is evanescent away from the interface. SPPs can be propagating along the interface, or they can be nonpropagating, i.e., spatially confined to, e.g., a plasmonic nanoparticle (localized surface plasmons polaritons) [243]. 

It has been shown that localized plasmon polaritons inthe region of sharp metal tips act in an analogous fashion, giving rise to TERS. In the usual mode of operation, TERS employs a sharp metal tip, which is illuminated from the outside to create a localized light source [42]. Alternatively, silver nanoparticles have been deposited on silica or titania surfaces and a silicon tip is used [43, 44]. TERS is rapidly becoming an important technique for microspectroscopy and is described in some detail in Chapter 11 of this book. 

As discussed in previous sections, plasmonic structures improve the absorption efficiency of the photovoltaic absorber layers by preferentially scattering [40] and exciting localized surface plasmons [12, 41] or plasmon polaritons [42-46]. Surface plasmons are localized by noble metallic nanoparticles (NPs) such as Cu [47-49], Ag and Au resulting in localized surface plasmon resonance (LSPR). Ag and Au NPs are the most widely used materials due to their surface plasmon resonances located in the visible range. A1 and Cu, which have resonance in the ultraviolet and... 

A.V. Zayats, I I. Smolyaninov, Near-field photonics Surface plasmon polaritons and localized surface plasmons, J. Opt. A Pure Appl. Op. 5 (2003) SI-S35. 

When a metallic probe, which has a nanometric tip, is illuminated with an optical field, conductive free electrons collectively oscillate at the surface of the metal (Figure 10.3). The quantum of the induced oscillation is referred to as surface plas-mon polariton (SPP) (Raether 1988). The electrons (and the positive charge) are concentrated at the tip apex and strongly generate an external electric field. Photon energy is confined in the local vicinity of the tip. Therefore, the metallic tip works as a photon reservoir. 

One may classify the various proposed models in several ways. One way is to differentiate between models that focus on the role of the electric field E and the emission G terms (these two are related), on the one hand, and those that emphasize the role of changes in the Raman polarizability tensor, on the other. The former discuss the enhancement in terms of amplified fields, due to the presence of the surface, which act on the scattering molecule and its emission being further amplified by the surface. These are the local field and emission enhancement models (LFE). The difference between the various models which belong to this group is in the identification of the specific excitation in the solid which is responsible for the amplification plasmon polaritons, shape resonances, electron holes, etc. 

If the thickness of the surface layer in which a surface exciton-polariton is localized considerably exceeds the lattice constant of a crystal, the electric and magnetic field strength vectors, i.e. vectors E and H of a wave with energy hw in both media (in vacuum and in the crystal the crystal is assumed to be nonmagnetic so that the magnetic induction vector B = H), satisfy Maxwell s equations... 

Collective optical excitations, like surface plasmon-polaritons in partially-ordered metal nanoparticle arrays, tend to be spatially localized. The localization facilitates a giant increase of linear and nonlinear optical responses such as Raman scattering, enhancement of spontaneous emission rate, nonlinear absorption and refraction. In this paper the spectral manifestation of light localization into metal-dielectric nanocomposites i s s tudied i n t he visible. T he e ffect o f t he 1 ateral e lectrodynamic coupling on transmission/reflection optical spectra is investigated for planar silver nanoparticle arrays (random close-packed and polycrystalline quasiregular structures). Combined action of electron and photon confinements is demonstrated experimentally and considered theoretically for ID-photonic crystals consisted of a metal nanoparticle stratified array. 

Figure 1. (a) Transverse local photonic DOS (%) for the two-level atom in the centers of the four zigzag CNs (x is the dimensionless frequency), (b) Two-particle local photonic DOS functions S (solid lines) and f (dashed lines) taken at the peak frequencies of if 00 [see (a)], as functions of the distances between the two atoms on the axes of the (10,0) (lines 1 x=0.29), (11,0) (lines 2 v=0.25) and (12,0) (lines 3 x=0.24) CNs (see Ref. [5] for more details), (c) Optical absorbtion lineshapes for the atom at different distances outside the metallic (9,0) CN, demonstrating the formation of the atomic quasi-ID polariton state as the atom approaches the CN surface (see Ref. [10] for more details), (d) Upper-level population decay probability of initially excited atom A (lines 1) and initially unexcited atom B (lines 2), and the two-qubit atomic entanglement (lines 3), as functions of dimensionless time r for the two atoms in the center of the metallic (9,0) CN separated from each other by the distance of 6.3/U = 22.2 A (see Ref. [6] for more details). 

We examined the effect of coupling between surface plasmon polariton (SPP) modes on the optical activity of metal nanostructures. By measuring the in-plane wave vector dependence on transmission and polarization azimuth rotation, we show that coupling of the SPP modes with orthogonal polarization localized at different interfaces is responsible for the optical activity in metal nanostructures. 

Instrumentation. In order to employ local enhancement of infrared absorption by surface plasmon polaritons that cause locally enhanced surface electromagnetic fields, a suitable optical arrangement is needed [295]. Surface enhanced infrared absorption spectroscopy can also be observed in the transmission mode [285, 296]. However, since no application of this approach in spectroelectrochemistry has been reported so far, it is not discussed further. 

As noted above, previous reports of the SEIRA effect had attributed the enhancement to a similar mechanism similar to the one leading to the SERS effect, namely the excitation of surface plasmon polaritons. " Because the effect was observed with Ni, Pt and Pd as well as Ag, Nakao and Yamada recognized that the effect that they observed was caused by some mechanism other than the effect of excitation of surface plasmon polar-itons. Nakao and Yamada postulated that the effect of multiple reflection in the metal film, of the decrease in penetration depth of the IRE caused by the metal layer and/or the effect of local (chemical) interaction at the metal-sample interface might contribute to the enhancement. However, as will be discussed later in this chapter, none of these putative causes fully explains the enhancement. 

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