Noble metal nanoparticles optical properties

Noble metal nanoparticles dispersed in insulating matrices have attracted the interest of many researchers fromboth applied and theoretical points of view [34]. The incorporation of metallic nanoparticles into easily processable polymer matrices offers a pathway for better exploitation of their characteristic optical, electronic and catalytic properties. On the other hand, the host polymers can influence the growth and spatial arrangement of the nanoparticles during the in situ synthesis, which makes them convenient templates for the preparation of nanoparticles of different morphologies. Furthermore, by selecting the polymer with certain favorable properties such as biocompatibiHty [35], conductivity [36] or photoluminescence [37], it is possible to obtain the nanocomposite materials for various technological purposes. 

An intense femtosecond laser spectroscopy-based research focusing on the fast relaxation processes of excited electrons in nanoparticles has started in the past decade. The electron dynamics and non-linear optical properties of nanoparticles in colloidal solutions [1], thin films [2] and glasses [3] have been studied in the femto- and picosecond time scales. Most work has been done with noble metal nanoparticles Au, Ag and Cu, providing information about the electron-electron and electron-phonon coupling [4] or coherent phenomenon [5], A large surface-to-volume ratio of the particle gives a possibility to investigate the surface/interface processes. 

In the first part, emphasis will be put on the linear optical properties of dielectric media doped with noble metal nanoparticles. Indeed, the study of the linear response is definitely needed to further explore the nonlinear one. We will then introduce the fundamentals of the theoretical tools required to understand why and how people inquire into the third-order nonlinear properties of nanocomposite materials. In the second part, experimental results will be presented by first examining the different nonlinear optical phenomena which have been observed in these media. We will then focus on the nanoparticle intrinsic nonlinear susceptibility before analysing the influence of the main morphological factors on the nonlinear optical response. The dependence of the latter on laser characteristics will finally be investigated, as well as the crucial role played by different thermal effects. 

LINEAR OPTICAL PROPERTIES OF NOBLE METAL NANOPARTICLES AND NANOCOMPOSITE MEDIA... 

Whatever the degree of approximation used in evaluating the effective nonlinear susceptibility of a composite medium, it can be seen in Eqs. (22), (23) or (27) that the result depends on the product of two complex quantities One linked with the medium morphology and composition (the local field factor), the other linked with the nonlinear optical properties of the metal inclusions themselves (the intrinsic third-order susceptibility, Xm ) - inasmuch as the own contribution of the host matrix to the whole nonlinear response still remains negligible. We will focus here on the second factor. It is noteworthy that very few theoretical work has been accomplished regarding the value of Xm for noble metal nanoparticles after the pioneering smdies of Flytzanis and coworkers [79, 80, 89, 90]. Moreover, as will be underlined below, their results may not be used in every experimental situation as they are. 

Electrically conductive polymers are perspective materials in modern technologies because of their potential applications as chemical sensors, catalysts, microelectronic devices, etc. [1]. The interest to new hybrid nanostructured materials based on polymer matrix with poly-7t-conjugated bonds and noble metals nanoparticles constantly increases. This is reasoned by a wide spectrum of new optical and electrophysical properties [2]. 

The optical absorption properties of noble metal nanoparticles in the visible range of the electromagnetic spectrum are determined by the effect of the boundary condition of the coherent electron oscillations as well as by d ip electronic transitions. Very small gold nanoparticles (d<2 nm), as well as bulk gold do not show a localized surface plasmon absorption band As discussed earlier (see Fig. 16.5), gold nanorods... 

Probing plasmonic resonances using low-loss electron energy loss spectroscopy has proved particularly useful for studying the optical properties of noble metal nanoparticles. Single particle plasmonic studies are an important tool to understand the role of size, shape and local environment on the exact nature of plasmonic excitations that might be complicated by dispersity of these traits when using bulk techniques. 

A large variety of synthetic procedures have been developed for the preparation of noble metal nanoparticles with various sizes and shapes, and which are stable in a wide range of solvents. The optical properties of these nanomaterials are currently quite well understood, since the quality of the current preparation methods allows to obtain a high monodispersity, so that simplified theories can be used. 

Section 1.2 outlined three major classes of nanofabrication techniques that are currently in use in laboratories around the world to produce noble metal nanoparticles. As previously discussed, noble metal nanoparticles are of particular interest due to their extraordinary optical properties. This section will review a number of fimdamental studies carried out to understand and exploit both the tunability of the resonance condition and the resultant amplification of the local electromagnetic fields. 

Noble metal nanoparticles can go beyond acting as labels recent advances show that changes in nanoparticle optical properties can act as the signal transduction mechanism in chemosensing and biosensing events. Van Duyne and coworkers have exploited the extreme LSPR sensitivity of NSL-fabricated nanoparticles to changes in local refiactive index in order to sense small molecules, amino acids, proteins, and antibodies. The LSPR shifts systematically to lower energies as the local dielectric constant increases ... 

In 1908, Mie proposed a theoretical model to explain the optical extinction (sum of the absorption and scattering properties) of noble metal nanoparticles. For nanoparticles with a radius (r) much smaller than the wavelength of light (2r X.), the extinction profile can be adequately explained by the simplified Mie formula (Eq.l). ... 

Gonzalez, A. L, Reyes-Esqueda, J. A., and Noguez, C. [2008] Optical properties of elongated noble metal nanoparticles,/ Phys. Chem. C, 112, 7356-7362. 

Metal nanoparticles have been studied mainly because of their unique optical properties especially nanoparticles of the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromagnetic spectrum. Solutions of these metal nanoparticles show a very intense color, which is absent in the bulk material and atoms. The origin of the intense color of noble metal nanoparticles is attributed to the collective oscillation of the free conductive electrons induced by an interacting electromagnetic field. These resonances are also denoted as siuface plasmons [15]. 

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