Local field enhancement

Interesting nanostructures, that may present an interaction among nanoclusters, with consequent increase of local field enhancement factor are obtained by irradiating AuCu alloy clusters with Ne ions at 190 keV [30]. 

While the linear absorption and nonlinear optical properties of certain dendrimer nanocomposites have evolved substantially and show strong potential for future applications, the physical processes governing the emission properties in these systems is a subject of recent high interest. It is still not completely understood how emission in metal nanocomposites originates and how this relates to their (CW) optical spectra. As stated above, the emission properties in bulk metals are very weak. However, there are some processes associated with a small particle size (such as local field enhancement [108], surface effects [29], quantum confinement [109]) which could lead in general to the enhancement of the fluorescence efficiency as compared to bulk metal and make the fluorescence signal well detectable [110, 111]. 

The next point to realize is that the best emitter is a metal. Many forms of carbon initially studied are semiconductors or even insulators, including nanodiamond [8-11] and diamond-like carbon (DLC) [12-13,4]. Combine this with local field enhancement means that there is never uniform emission from a flat carbon surface, it emits from local regions of field enhancement, such as grain boundaries [8-11] or conductive tracks burnt across the film in a forming process akin to electrical breakdown [13]. Any conductive track is near-metallic and is able to form an internal tip, which provides the field enhancement within the solid state [4]. Figure 13.2 shows the equipoten-tials around an internal tip due to grain boundaries or tracks inside a less conductive region. 

The frequency dependence of SHG at simple metal surface has been the focus of a recent theoretical study of Liebsch [100]. Time-dependent density functional theory was used in these calculations. The results suggest that the perpendicular surface contribution to the second harmonic current is found to be significantly larger than had been assumed previously. He also concludes that for 2 a> close to the threshold for electron emission, the self-consistently screened nonlinear electronic response becomes resonantly enhanced, analogous to local field enhancement in the linear response near the bulk plasma frequency. 

Bouhelier A, Beversluis M, Flartschuh A, Novotny L (2003) Near-field second-harmonic generation induced by local field enhancement. Phys Rev Lett 90 013903... 

Kumar LKS, Gordon R (2006) Overlapping double-hole nanostructure in a metal film for localized field enhancement. IEEE J Sel Top Quantum Electron 12 1228-1232... 

Depth can be an indicator of interactions between LSPs and bulk SPs. In thick nanowires, an LSP is well isolated from bulk SPs, while LSPs interact with and are affected from bulk SPs significantly in shallow nanowires. It was shown that penetration depth was calculated to be smaller for thicker nanowires, as a result of increased field localization [17]. In fact, a nanowire period A) and a fill factor (/) should also be considered in this picture, in the sense that LSPs tend to be coupled to each other at appropriate values of A and/. On the one hand, if LSPs perturb SPs weakly, i.e., LSPs are dominated by SPs, local field enhancement and sensitivity improvement over conventional SPR structure are rather weak. On the other hand, if LSPs dominate SPs, resonance characteristics become so broad that resonance effectively disappears. In short, an optimum nanowire depth exists, although the optimum may depend on other parameters. 

Nanostructure-based LSPR biosensing has been classified into three schemes surface-relief nanostructures, surface-relief nanostructures coupled to nanoparticles, and nanoparticles. These schemes share many aspects of plasmon characteristics in common, such as shape and concentration dependence. Local field enhancement as a result of plasmon excitation can be used for highly sensitive biosensing. Given particular bio-sensing applications, one of these schemes can be selected to meet... 

The local field enhancement factor, characterizing the modification of absorption. This results, in most cases of interest, in an enhancement, and possibly a very large enhancement up to = 10. ... 

Moreover, it is possible to show that the local field enhancement factor and the radiative enhancement factor Mnjj(ty) are in many cases... 

Moreover, for the MEF signal to be observable, it is also necessary that the local field enhancement at the laser fiequency loc( /,) is sufficiently large. Since and follow approximately the same resonant profile,... 

A larger modification is predicted when the resonance occurs beyond the free-space fluorescence peak (case D of Fig. 2.2(b)). The MEF signal is however much smaller because of the small local field enhancement factor at. ... 

The situation is similar in the case of a gold sphere, because the radiative (and local field) enhancements remain small compared to A/.j., for... 

The incident polarization only affects the coupling to the LSP resonances at the laser frequency, i.e. the local field enhancement factor at... 

To understand the importance of spectral overlap to metal-enhanced fluorescence, it is useful to review the basics of metal-enhanced fluorescence. Metal nanostructures can alter the apparent fluorescence from nearby fluorophores in two ways. First, metal nanoparticles can enhance the excitation rate of the nearby fluorophore, as the excitation rate is proportional to the electric field intensity that is increased by the local-field enhancement. Fluorophores in such "hot spots" absorb more light than in the absence of the metal nanoparticle. Second, metal nanoparticles can alter the radiative decay rate and nonradiative decay rate of the nearby fluorophore, thus changing both quantum yield and the lifetime of the emitting species. We can summarize the various effects of a nanoparticle on the apparent fluorescence intensity, Y p, of a nearby fluorophore as ... 

Chen, C. J. and Osgood, R. M. (1983). Direct observation of the local-field-enhanced surface photochemical reactions. Phys. Rev. Lett. 50 1705-1708. 

As already mentioned for aperture arrays, the periodicity of the structure can lead to significant local field enhancement by resonantly exciting surface plasmons. To translate this resonance effect to an isolated aperture, the metal surface surrounding the ajjerture can be structured in a periodic maiuier in order to efficiency excite the SPP. Most designs use concentric grooves around a central nanoajjerture, which is called bull s eye aperture (56, 70, 71). 

Shalaev, V.M. (2001). Fractal nano-composites giant local-field enhancement of optical responses, In Nanoscale linear and nonlinear optics, Bertolotti, M. and Sibilia, C. (Ed.) AIP. 

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