Morphology and plasmonic properties of self-organized arrays of ...

92 CHAPTER 5. MODELLING AND ANALYSIS OF THE OPT. PROP.55Re[]432103xxε effyyε effzzε effRe[]43213Im[]2Im[]211020040060080010001200020040060080010001200 [nm] [nm]Figure 5.13: Real and imaginary parts of the computed principal components of theeffective dielectric tensor for the square (left panel) and rectangular (right panel) samples.of the imaginary part. It should therefore be expected for the LSP peaks observed inreflection to be red-shifted with respect to the corresponding minima of transmissivity.In order to better clarify the effect of the NPs shape dispersion, the R S and R P spectracalculated in absence of L spread are also reported in fig. 5.11 and fig. 5.12 as the thindashed lines. Comparing these curves to the ones calculated for σ L > 0, we can see that,introducing a dispersion of depolarization factors, both the L and T LSP modes redshift,and the resonances become weaker and broader. These effects are especially pronouncedfor the L mode of the square sample, i.e. for elongated particles along the major axis, whilethe corresponding T LSP peak is only slightly modified; for the rectangular sample thefinite dispersion effects are instead equally observed for all the involved plasmon modes.DiscussionIn order to fully exploit the possibility of engineering the plasmonic response of the selforganizedarrays, the intrinsic single particles properties and the effects of the dipolarcoupling on the LSP resonances must be clearly highlighted. In particular, we are interestedin understanding how the parameters of fabrication of our samples (substratetemperatures, thickness of deposited gold) affect the LSP, and the relative weights ofthe intrinsic and collective effects on the optical anisotropy. Calculating the evolution ofthe LSP peak wavelength as a function of increasing array dimensionality, i.e. graduallyintroducing mutual interactions in the system along and across the ripples, provides aparticularly simple way to achieve this understanding.We therefore computed the optical response of the square and rectangular samples forthree steps of increasing dimensionality: no EM coupling between NPs (0D, equivalent toisolated NPs), EM coupling between NPs allowed only within the single chains (1D, wherea “chain” is defined as being oriented along the LiF ridges), and EM coupling between allNPs in the array (2D). The variation in the (theoretical) position of the LSP will provideinteresting clues about the mechanism of EM coupling in the system.