Metal nanospheres

Figure 3.8 Schematic view of enhanced field distribution in the vicinity of a dimerof noble metal nanospheres. (Reproduced with permission from The Japan Society of Applied Physics [12]).

This chapter is devoted to describe the impact of metallic nanosphere to the multi-photon excitation fluorescence of Tryptophan, and little further consideration to multi-photon absorption process will be given, as the reader can find several studies in [11-14]. In section II, the nonlinear light-matter interaction in composite materials is discussed through the mechanism of nonlinear susceptibilities. In section III, experimental results of fluorescence induced by multi-photon absorption in Tryptophan are reported and analyzed. Section IV described the main results of this chapter, which is the effect of metallic nanoparticles on the fluorescent emission of the Tryptophan excited by a multi-photon process. Influence of nanoparticle concentration on the Tryptophan-silver colloids is observed and discussed based coi a nonlinear generalization of the Maxwell Garnett model, introduced in section II. The main conclusion of the chapter is given in secticHi IV. 

Li KR, Stockman MI, Bergman DJ (2003) Self-similar chain of metal nanospheres as an efficient nanolens. Phys Rev Lett 91(22) 227402... 

Norton SJ, Vo-Dinh T (2008) Optical response of linear chains of metal nanospheres and nanospheroids. J Opt Soc Am A 25(ll) 2767-2775... 

Fig. 9.2. Cartoon showing the seed-mediated growth approach to the synthesis of metallic nanorods of controllable aspect ratio. In the first step, metal salts are reduced with sodium borohydride, a strong reducing agent, to metal nanospheres ( seeds ) that are 3-4 nm in diameter. In the subsequent growth steps.

If the shape of metal nanoparticles is close to spherical, one can get the single scattering characteristics by the Mie theory with taking into account size dependence of metal permittivity [2]. For close-packed nanoparticle arrays it is mosdy convenient to use the quasicrystalline approximation (QCA) of the STMSW, which reduces the required structural information to the radial distribution function. The QCA approach, employed in this research has already been applied to 2D close-packed metal nanosphere arrays [3,6] and has shown to be in good agreement with experimental data [6]. 

The electromagnetic enhancement mechanism features the major contribution to the overall enhancement of SERS. It is based on the generation of an electromagnetic field at the surface of nanostructured metal surfaces due to the interaction of an incident electromagnetic field and the excitation of localized surface plasmon polaritons. To explain this phenomenon in more detail, a simple model can be used. A simple metal nanosphere with a size smaller than the wavelength of the incident light is considered for this purpose. This metal nanosphere is surrounded by a medium or vacuum with a dielectric constant Eq, and all appearing processes are assumed to be quasi-static. The dielectric constant inside the metal nanosphere is independent of the size of the sphere and is described as follows ... 

If an incident laser light Eo interacts with the metal nanosphere, a collective movement of the electrons against the atomic cores of the metals - a so-called surface plasmon - is induced. The interaction of the surface plasmons and the incident laser light causes the so-called localized surface plasmon polaritons, resulting in an evanescent electromagnetic field E y, which is emitted fi-om the nanoparticle. The signal intensity achieved for SERS depends on the absolute square of the emitted field Egy, which can be simplified written as follows ... 

Finally, the new field of nanotechnology is now penetrating into biophotonies. Examples include the use of nanoparticles such as metal nanospheres or rods and quantum dots for enhanced cell and tissue imaging and local light energy absorption. The ehapter by C.E. Talley et al. discusses one specific implementation, namely the use of nanoparticles for enhancing Raman biospectroscopy. 

A. A. Lazarides and G. C. Schatz, DNA-linked metal nanosphere materials Structural basis for the optical properties, J. Phys. Chem. B 104(3), 460-467 (2000). 

The basic problem of nonlocal theories is to find an appropriate e(,k,co). Several authors have dealt with this problem for both geometries, a dipole above a surface and a dipole close to a metal nanoparticle.It is certainly beyond the scope of this chapter to go into detail of those theories. However, let us briefly note that the results are miscellaneous. For example, in the case of a dipole close to a metal nanosphere, Leung predicts one to two orders of magnitude less energy transfer to the nanoparticle in the case where the dipolar transition is energetically lower than the particle plasmon resonance. Ekardt and Penzar predict exactly the opposite. 

We now consider a simple application for the scattering formalism introduced above. We consider a dielectric or a metal nanosphere in the vacuum, centered at the fq = 0 and excited by a a plane-wave with Sine given by Eq. (1.282). If the sphere is very small R -C A.) we have the so called Rayleigh scattering and we can use the quasi-static solution of Sec. 1.4.1. In particular we can assume that sphere can be modeled by a dipole... 

We conclude this first chapter with the most important case in molecular plasmonics an emitting dipole near a metallic nanosphere. For this problem the exact electrod5mamic solution can be obtained, as shown by Ruppin [41], Chew [42] and by Kim et al. [43]. 

These equations have been also obtained by Carminati et al. [17] and directly show a d dependence at large nanoparticle-molecule distances. In the Carminati model the metal nanosphere is considered as a point-like (located in rp) polarizable entity with polarizability (i.e. due to radiative damping, see Sec. 1.6.4). Thus... 

Mertens, H., Koenderink, A. E, and Polman, A. (2007) Plasmon-enhanced luminescence near noble-metal nanospheres Comparison of exact theory and an improved Gersten and Nitzan model, Phys. Rev. B, 76, 115123/1-12. 

Figure 5.3 Synthesis of silica nanoparticles within self-assembled organo-metallic nanospheres. Reprinted with permission from Ref. (20). Copyright 2010 Macmillan...

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