ABSTRACT
Gold (Au) and silver (Ag) are the most commonly used materials in nanoSPR since they exhibit strong SPRs in the visible-near-infrared (NIR) wavelength range [32]. At these wavelengths, the dielectric properties of the metals are described by a complex, wavelength-dependent dielectric coefficient, e e(l) = er(l) + iei(l) (3.1)where e = m2 and m = n + ik is the complex refractive index given as a function of the refractive index n and the absorption coefficient k[20]. Noble metal nanoparticles can
support nanoSPR [33] when the excitation photon frequency resonates with the collective oscillation of the conduction electrons confined inside the nanoparticles (Fig. 3.1) [34]. In the case of nanoSPR, the size of the particles (d) is much smaller than the wavelength of the incident light (d << l). The conduction electrons inside the nanoparticles move in phase-up plane-wave excitation. This leads to the buildup of polarization charges on the nanoparticle surface, allowing a resonance to occur at a specific frequency (the nanoparticle dipole plasmon frequency) [33, 35]. A homogeneous field builds up in the interior of the nanoparticle, while a dipolar field is produced at the exterior, resulting in strong light scattering, the appearance of intense SPR bands, and the enhancement of the nearfield in the immediate vicinity of the nanoparticles. The spectroscopic responses of larger metallic nanoparticles are different due to the excitation of higher-order modes such as quadrupoles and retardation and skin-depth effects [33, 35]. The bandwidth, peak height, and peak wavelength all depend on the nanoparticle chemical composition, size, and geometrical shape (Fig. 3.2) and the dielectric property of the surrounding environment [33, 36, 37].
