Electromagnetic Field Enhancement

Energy conservation suggests that when the reflectivity at SP excitation is minimal, the intensity of the SP electromagnetic field has its maximum. For the Kretschmann configuration, the intensity enhancement, tj, which is obtained in SP is given by the ratio of the SP intensity in medium 1 to the intensity of light incident onto the prism/film interface, ft is found as (Raether 1988) 

It should be noted, however, that if one calculates the enhancement factor relative to the intensity of the light before it enters the prism, i.e., in the surrounding medium 1, r has to be exchanged by ei/n )t] and thus the enhancement is somewhat smaller. 

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M. (2006) Ahgned silver nanorod arrays for surface-enhanced Raman scattering. Nanotechnology, 17, 2(>70-2(>74, (b) Lu, Y, Liu, G.L., Kim, J., Mejia, Y.X. and Lee, L.P. (2005) Nanophotonic crescent moon structures with sharp edge for ultrasensitive biomolecular detection by local electromagnetic field enhancement effect Nano Letters, 5, 119-124 ... 

Nordlander, P., and Le, F. 2006. Plasmonic structure and electromagnetic field enhancements in the metallic nanoparticle-film system. Appl. Phys. B 84 35 1. 

The electromagnetic field enhancement provided by nanostructure plasmonics is the key factor to manipulate the quantum efficiency. However, as it is illustrated in the unified theory of enhancement, since both the radiative and non-radiative rates of the molecular systems are affected by proximity of the nanostructure, the tuning has to be done on a case by case basis. In addition, there are factors due to molecule-metal interactions and molecular orientation at the surface causing effects that are much more molecule dependent and as are much more difficult to predict. Given the fact that fluorescence cross sections are the one of the highest in optical spectroscopy the analytical horizon of SEF or MEF is enormous, in particular in the expanding field of nano-bio science. 

Theoretical and experimental studies demonstrated that the electromagnetic field enhancement is substantially increased at surface sites having local geometric singularities. In particular, enormous field enhancement occurs at the junction between two or more almost touching nanoparticles [5-7] due to plasmon coupling. For spacings of 1-2 nm, maximum enhancement factors of about lO -lO were calculated [6, 8]. 

Lu Y, Liu GL, Kim J, Mejia YX, Lee LP (2005) Nanophotonic crescent moon structures with sharp edge for ultrasensitive biomolecular detection by local electromagnetic field enhancement effect. Nano Lett 5 119-124... 

The application of the ATR method in Raman spectroscopy provides a unique way to study essentially mirrorlike polished metals, in particular single crystalline surfaces. The ATR method is used to excite surface plasmon polaritons (SPPs) effectively at the smooth metal surface, to improve the sensitivity in Raman spectroscopy by electromagnetic field enhancement [19]. The enhancement of the ATR configuration, as shown in Figs. 8(d and e), with respect to the normal external reflection geometry spans one to three orders, depending on the electrodes and their crystallographic orientations. 

This hypothesis was supported by normal Raman and SERS spectra of adenine. Assuming that the electromagnetic field enhancement was constant during the measurements, it is reasonable that chemical and mechanical enhancement mechanisms were responsible for band fluctuations. These mechanisms require a very small distance between the TERS tip and the molecule, as realized in contactmode TERS. 

From Eq. (33.6), it is clear that the SPR wavelength position depends on the optical characteristics of the surrounding media and consequently if the environment changes or has fluctuations, the plasmon resonance will shift its location. The Frolich condition is directly associated with an electromagnetic field enhancement that takes place in the vicinity of metallic NP due to the collective excitations of the electrons. These concepts are the first stones in the use of plasmonic devices as sensors, as it is the case of svuface-enhanced Raman spectroscopy. Ordered OMPO structures filled with silver NPs are suitable substrates for SERS as they give a reproducible enhancement over a wide surface area (Figure 33.8b and c). 

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