Plasmon electric field enhancement

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

In this chapter, we have provided an overview of near-field imaging and spectroscopy of noble metal nanoparticles and assemblies. We have shown that plasmon-mode wavefunctions and enhanced optical fields of nanoparticle systems can be visualized. The basic knowledge about localized electric fields induced by the plasmons may lead to new innovative research areas beyond the conventional scope of materials. 

FIGURE 10.3 Quantized oscillation of electrons at the surface of a metallic probe tip. This is so called surface plasmon. As the charge distribution is confined tightly at the sharp tip end, the subsequent electric field at the tip is strongly enhanced. 

One simple explanation for these results was as follows The electric field at a metal vacuum interface can be >10 times larger than in free space when the conditions required for a surface plasma resonance are met (47). Since the Raman cross-section is proportional to the square of the field, surface plasmons could produce enhancements of >10. This enhancement is probably not large enough to explain the tunneling junction results by itself, but an enhancement in signal of a factor of 100 by the excitation of surface plasmons would increase the Raman intensity from near the limits of detectibility. 

A complementary approach to the standard reflection geometry described above uses the attenuated total reflection (ATR) geometry which couples surface plasmon waves to the incident electric field and enhances the SH production. Two configura-... 

The detected fluorescence can be significantly enhanced, however, by exploiting the plasmonic enhancement which can occur when a metal nanoparticle (NP) is placed in the vicinity of a fluorescent label or dye [1-3]. This effect is due to the localised surface plasmon resonance (LSPR) associated with the metal NP, which modifies the intensity of the electromagnetic (EM) field around the dye and which, under certain conditions, increases the emitted fluorescence signal. The effect is dependent on a number of parameters such as metal type, NP size and shape, NP-fluorophore separation and fluorophore quantum efficiency. There are two principal enhancement mechanisms an increase in the excitation rate of the fluorophore and an increase in the fluorophore quantum efficiency. The first effect occurs because the excitation rate is directly proportional to the square of the electric field amplitude, and the maximum enhancement occurs when the LSPR wavelength, coincides with the peak of the fluorophore absorption band [4, 5]. The second effect involves an increase in the quantum efficiency and is maximised when the coincides with the peak of the fluorophore emission band [6]. 

Of the two different types of plasmonic enhancement which were described in section 6.1, the emphasis here is on the excitation enhancement mechanism. There is a linear dependence of the excitation rate of a fluorescent dye on the intensity of the excitation light in the direction of the electric dipole, e, of the molecule. When the dye molecule is located near the NP, the electric field acting on the dipole changes from Ei to Ej + E,. In this case, the excitation enhancement factor, fjoh. for one dye molecule is defined as a ratio of intensities ... 

Dye molecule orientation at the surface of a NP has a significant influence on the enhancement factor. For an ideal conductor, the electric field is always perpendicular to the surface. As discussed above, in order to demonstrate plasmonic enhancement as a function of NP radius, NPs were coated with a silica shell prior to attachment of a dye in order to minimize dye quenching. The presence of this shell slightly modifies the electric field but it is still almost perpendicular to the surface. It follows from this and from Equation 4 that molecules, which aie oriented normally, are considerably more excited than those oriented tangential to the surface. For this model, we assume that there is random dipole orientation at the NP surface. The average intensity over all possible dipole positions due to a EM field E is given by 1/3 Ef. Hence the averaged enhancement factor can be written as ... 

Highly enhanced local fields can be generated in the vicinity of a metallic spherical nanoparticle due to localized surface plasmon excitation. Using the result in Eq. (10b), the electric field components at the sur ce of the small sphere can be... 

To optimize local enhancement of the electric field we need to minimize all damping as much as possible and the suitability of certain nanoparticle morphologies for MEF by increased excitation of fluorophores can be estimated from measurements of the homogeneous line width of individual nanoparticles. For example, a series of expoiments comparing nanospheres and nanorods (see Figure 11.14) has shown that nanorods typically display dramatically reduced plasmon damping compared to spheres, i.e. narrower line vridths [34], and therefore produce a stronger field enhancement. 

The field reaches its maximum at the surface plasmon resonance frequency when e = -2 Co where Co is the dielectric constant of the medium surrounding the particle surface. This induced field of the metallic nanoparticies provides an external field for the fluorescence excitation of the molecules in addition to the electric field of the incident light and thus increases the absorption rate which is responsible for the enhanced fluorescence intensity. 

There are two types of SERS mechanisms, which are responsible for the observation of the SERS enhancement [8] one type is the long-range EM effect and the other is the short-range CHEM effect. The EM effect is believed to be the result of localized surface plasmon resonance electric fields (hot spot) set up onto the roughened metallic surfaces [9, 10, 31]. The probe molecules residing within these hot spots will be strongly excited and subsequently emit amplified Raman... 

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