Metallic nanoparticles local field enhancement

Fluorescence and Enhanced Local Field of Metallic Nanoparticles... 

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,... 

This factor is taken into account in (3.5)-(3.8) above, but it can have an even greater importance in nonlinear effects, since the second-order and third-order nonlinear optical coefficients, and respectively, are affected by factors and /, respectively, as compared with the bulk material of the nanoparticle. Hence, for large /, a nanostructured material can have a larger optical nonlinearity than its bulk constituents. For typical semiconductor-doped matrices, > and /< 1. However, particularly strong local-field enhancements are observed for metal nanoparticles in the vicinity of the plasmon resonance [3.75]. 

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. 

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. 

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