Metallic nanoparticles enhanced local field

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... 

Fluorescence and Enhanced Local Field of Metallic Nanoparticles... 

Metal nanoparticles present localized surface plasmon resonances (LSPRs) that are collective excitations of the electrons at the interface between a conductor and a dielectric. The resonant plasma oscillation causes local field enhancement, and this is utilized in SERS [61,62], second-harmonic generation [63], and scanning near-field optical microscopy [64]. In particular, certain metals such as silver and gold have been much studied due to the feet that they present this LSPR in the visible spectral region. 

One possible explanation for this enhanced energy transfer could be related to the nature of polymer excitons and the fact that the Si02 Au nanoparticles can exhibit greatly enhanced local field intensities. Since metal nanoparticle resonances have excitation lifetimes of only a few picoseconds, the donor-... 

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. 

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 ... 

One may ask why these experiments only showed quenching and not enhancement. The first thing to take note is the fact that the metal nanoparticles here are spherical, and therefore the SPR does not produce a very large enhancement of the local field. Also, the nanoparticles are small, which, as will be explained in Section 11.3,... 

An increase in excitation of the fluorophore depends on the spectral overlap between the SPR and the excitation spectrum of the molecule and on the enhancement of the local field which, as can be seen below, depends on the position of the fluorophore and its distance from the metal surface. The distribution of the local (enhanced) fields for a nanoprism and nanorod are illustrated in Figure 11.11 [S]. The largest field intensities occur at the tips of the nanoprism and at the ends of the nanorods. The field intensities are calculated to be approximately 4000 times the applied field. These field enhancements are much larger than can be obtained with spheres. Even larger field oihancements can be obtained at the interface of nanoparticles in very close proximity to one another, as shown in Figure 11.12 [5]. 

Metal nanoparticles have attracted considerable interest due to their properties and applications related to size effects, which can be appropriately studied in the framework of nanophotonics [1]. Metal nanoparticles such as silver, gold and copper can scatter light elastically with remarkable efficiency because of a collective resonance of the conduction electrons in the metal (i.e., the Dipole Plasmon Resonance or Localized Surface Plasmon Resonance). Plasmonics is quickly becoming a dominant science-based technology for the twenty-first century, with enormous potential in the fields of optical computing, novel optical devices, and more recently, biological and medical research [2]. In particular, silver nanoparticles have attracted particular interest due to their applications in fluorescence enhancement [3-5]. 

Figure 19.1 (A) 2D projection of the calculated local field intensity distribution around a pair of 15 nm diameter silver nanoparticles excited with Xi = 400 nm light polarized along the interpaiticle axis. The edge-to-edge particle separation is 2 nm and the free space incident light intensity Ej,x P taken to be unity. The local field intensity near the pair is shown in false color. The calculation was done using dipole-dipole approximation (DDA) method with each dipole unit being a square with sides of 0.2 nm. (B) Model of the photophysics of a molecule represented by a three level system and how the excitation and decay dynamics are affected by plasmon enhancement of radiative rates and the introducticm of a rate for quenching Icq of the excited state due to proximity to the metal surface. E (X ) and E (X2) are the field enhancements at the position of the molecule for the excitation and emission wavelengths respectively, kn and kMR represent the radiative and non-radiative decay rates of the molecule in the absence of plasmon enhancement.

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