Scattering cross section maps (the absorption cross sections always being zero) again give guidelines
for an adequate radius in order to obtain the main scattering resonance at λ approximately 700 nm click here (see Additional file 2: Figure S2). This requirement is fulfilled for the dielectric nanoparticle (in air) with n = 2, k = 0 for a radius of 170 nm which is distinctly larger than in the case of metallic nanoparticles (r = 120 nm). Figure 4a represents the total scattering cross section with the main resonance around 700 nm together with the division into the different order electromagnetic modes which are manifold for this medium-sized nanoparticle. As Figure 4a shows, the magnetic modes dominate the peaks of the scattering cross section and the electric modes contribute in the form of a broader background. The maximum scattering cross section reaches a value of nearly 6 which is the same as for the 120-nm radius Drude-fitted Ag nanoparticle. From this point of view, the dielectric nanoparticles appear to perform equally well or, considering the zero absorption, even better than the metallic ones. Looking at the near fields of the dominant resonance modes (Figure 4b), however reveals distinct differences: the magnetic modes of the dielectric nanoparticles appear to localize
the electromagnetic field inside the particle and the direction of light extraction seems to be preferential to the direct forward direction, i.e., the dielectric nanoparticle appears like a lens. There is a strong near field in this direction in contrast to the remaining selleckchem surface of the nanoparticle. We will come back to a detailed comparison of the angular distributions of the scattered light in a later section. Here, we only record that dielectric nanoparticles
are characterized by a strong scattering, yet not by a pronounced near field enhancement around the particle. Figure 4 Scattering and near fields of a dielectric nanoparticle. (a) Scattering cross section of a 170-nm radius nanoparticle with refractive index n = 2 and k = 0; sum and allocation to different order and electromagnetic (E/M) modes. (b) Near field Ro 61-8048 distribution of the electromagnetic field around the nanoparticle for the dipole, the quadrupole, the hexapole, and Bay 11-7085 the octopole magnetic mode at wavelengths of 700, 502, 392, and 322 nm, respectively, which correspond to the maxima in scattering (incident light from the top). Semiconductors After having seen both the benefits of the metallic as well as of the dielectric nanoparticles, we move on to considering nanoparticles of semiconductor material which might combine the two particular properties of free charge carriers and an area of approximately zero absorption. In the case of a semiconductor, furthermore, its band gap needs to be considered which can be achieved using the Tauc-Lorentz combined density of states and an oscillator model.