2.1.1 Nanoparticles on the Front and Rear of Substrates We have made corrections to the normalisation of the incident field in the calculation of the driving field at the position of the particle. For particles on the front Ed is a superposition of the incident field (Ei) and the field reflected (Er) from the air/SiO2/Si layer structure; for particles on the rear Et is equal to the field transmitted through the Si/SiO2 /air layer (Et). Previously we defined EiN as the incident field normalised to the refractive index in each medium, so that the ratio between EiN in the air region (front- located particles) and in the Si region (rear-located particles) is equal to nSi, the complex refractive index of Si. However, if the incident irradiance, given by  (Formula Presented) is the same in both mediums, we get the following relationship for the incident fields: (Formula Presented) The magnitude of the driving field at resonance, for rear-located particles, is then a factor of (Formula Presented) larger than previously calculated, as shown in the corrected Fig. 2.16b. For front-located particles, in Fig. 2.16a, |Ed|/|EiN| is unchanged by the correction. As before, the normalised driving field increases with spacer layer thickness for particles on both the front and the rear of the silicon substrate. Corrected Fig. 2.15 shows the Qscat(?spr) plotted against the corresponding (|Ed|/|EiN|)2 for layer thicknesses from 0 to 45 nm. As before, there is a clear correlation between Qscat (λspr) and (|Ed|/|EiN|)2 for front-located particles. Additionally, for rear-located particles on relatively thick spacer layers of 10 nm or larger, small increases in the driving field intensity correspond to slight increases in Qscat (λspr). However, for rear-located particles on ultrathin spacer layers of less than 10 nm, Qscat (λspr) increases rapidly as the spacer layer thickness reduces, despite the fact that the change in the driving field is small. This is likely to be due to changes in the near field for ultrathin spacer layers. It should be noted that, by using the corrected values for (|Ed|/|EiN|)2, the asymmetry in the driving field does not fully account for the asymmetry in the scattering cross section; in the corrected Fig. 2.15 the scattering cross section for rear-located particles is lower than that of the front-located particles for similar calculated driving field intensities. We attribute this to differences in the efficiency in the excitation of the plasmonic resonances for finite particles, when illuminated from the air region or the Si. Nevertheless, it is clear that optimising the driving field is important when achieving strong scattering from front-located particles on Si substrates. Additionally, rear- located particles on ultrathin spacer layers exhibit anomalous enhancement in scattering cross sections, which can be exploited to enhance the light trapping efficiency of plasmonic nanoparticle arrays.
|Title of host publication||Nanotechnology in Australia|
|Subtitle of host publication||Showcase of Early Career Research|
|Publisher||Pan Stanford Publishing|
|Number of pages||3|
|Publication status||Published - 2011|