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

4.2. OPTICAL PROPERTIES OF AU NANOPARTICLES ARRAYS 670.80.7T0.60.5||0.4a.0.34006008001000 [nm]120014001600T ||1.0T ┴0.90.8T0.70.6||0.5b.0.44006008001000 [nm]120014001600Figure 4.7: Transmissivity spectra measured at normal incidence on a nanopatternedLiF(110) (Λ ≈ 25 nm) after the deposition of t Au ≈ 5 nm gold at T = 100 ◦ C (panel (a))and subsequent annealing at T = 400 ◦ C (panel (b)). The transmissivities were acquiredwith linearly polarized light, parallel (red curves) and perpendicular (black curves) to theLiF ridges.gold grains. In the direction perpendicular the ripples, the grains are well separatedby the ripples ridges, so the electrons are effectively confined and the LSP peaks quitesharp. On the contrary, inside each groove the Au grains can accommodate as close aspossible to one another, and there are no constraints on their dimensions; the broadeningof the resonances, when the electric field is parallel the ripples, can then be attributedin part to the dispersion of the grains lengths, so that the observed peak is actually thesuperimposition of several peaks slightly shifted in wavelength, in part to the coalescenceof several contiguous grains, which reduces the electronic confinement restoring a metallicbehaviour somewhat analogous to a bulk sample. Thus, we will refer to this class ofsamples as gold “nanowires”.Having measured the reflectivities, we can now give a qualitative interpretation of theΨ spectra, which are roughly equivalent to the ratio R P /R S between the reflectivities;we consider only the region of interests λ 500 nm. When the plane of incidence isperpendicular to the ripples, Ψ (fig. 4.4(b)) corresponds to the ratio between RP ⊥, peakedat λ ≈ 600 nm, and RS ⊥, slowly varying, so it has a sharp peak similar to R⊥ P but shiftedat a different wavelength, because RS ⊥ is not a constant. On the contrary, Ψ as measuredin || geometry (fig. 4.4(a)) is roughly the ratio between a nearly constant curve (R || P )